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Read the following passage, and mark the letter (A, B, C or D) on your answer sheet to indicate the correct answer to each question.
 The first scientific attempt at coaxing moisture from a cloud was in 1946, when scientist Vincent Schaefer dropped 3 pounds of dry ice from an airplane into cloud and, to his delight, produced snow. The success of experiment was modest, but it spawned optimism among farmers and ranchers around the country. It seemed to them that science had finally triumphed over weather.
Unfortunately, it didn’t work out that way. Although there were many cloud- seeding operations during the late 1940s and the 1950s, no one could say whether they had any effect on precipitation. Cloud seeding, or weather modification as it came to be called, was clearly more complicated than had been thought. It was not until the early 1970s that enough experiments had been done to understand the processes involved. What these studies indicated was that only certain types of clouds are amenable to seeding. One of the most responsive is the winter orographic cloud, formed when air currents encounters mountain slope and rise. If the temperature in such a cloud is right, seeding can increase snow yield by 10 to 20 percent.
There are two major methods of weather modification. In one method, silver iodide is burned in propane- fired ground generators. The smoke rises into the clouds where the tiny silver iodide particles act as nuclei for the formation of ice crystals. The alternate system uses airplanes to deliver dry-ice pellets. Dry ice does not provide ice- forming nuclei. Instead, it lowers the temperature near the water droplets in the cloud so that they freeze instantly – a process called spontaneous nucleation. Seeding from aircraft is more efficient but also more expensive.
About 75 percent of all weather modification in the United States takes places in the Western states. With the population of the West growing rapidly, few regions of the world require more water. About 85 percent of the waters in the rivers of the West comes from melted snow. As one expert put it, the water problems of the future may make the energy problems of the 70s seem like child’s play to solve. That’s why the U.S. Bureau of Reclamation, along with state governments, municipal water districts, and private interests such as ski areas and agricultural cooperatives, is putting increased efforts into cloud- seeding efforts. Without consistent and heavy snowfalls in the Rockies and Sierras, the West’s disastrous snow drought of 1976-77. It is impossible to judge the efficiency of weather modification based on one crash program, but most experts think that such hurry- up programs are not very effective.
It can be inferred from the passage that the weather-modification project of 1976-77 ___________.
A.put together quickly
B.a complete failure
C.not necessary
D.easy to evaluate
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Các câu hỏi liên quan

 Read the following passage, and mark the letter (A, B, C or D) on your answer sheet to indicate the correct answer to each question. 
 Visitors to Prince Edward Island, Canada, delight in the “unspoiled” scenery – the well-kept farms and peaceful hamlets of the island’s central core and the rougher terrain of the east and west. In reality, the Island ecosystems are almost entirely artificial.
Islanders have been tampering with the natural environment since the eighteenth century and long ago broke down the Island’s natural forest cover to exploit its timber and clear land for agriculture. By 1900, 80 percent of the forest had been cut down and much of what remained had been destroyed by disease. Since then, however, some farmland has been abandoned and had returned to forest through the invasion of opportunist species, notably spruce. Few examples of original climax forest, which consisted mostly broadleaved trees such as maple, birch, and oak, survive today.
Apart from a few stands of native forest, the only authentic habitats on Prince Edward Island are its sand dunes and salt marshes. The dunes are formed from sand washed ashore by waves and then dried and blown by the wind to the land beyond the beach. The sand is prevented from spreading farther by marram grass, a tall, long- rooted species that grows with the dunes and keeps them remarkably stable. Marram grass acts as a windbreak and allows other plants such as beach pea  and bayberry to take hold. On dunes where marram grass is broken down- for instance, where it is trampled- the dunes may spread inland and inundate agricultural lands or split up fishing harbors. The white dunes or the north coast are the most impressive. There are also white dunes on the east and west coasts. Only in the south are there red dunes, created when soft sandstone cliffs crumble into the sea and subsequently wash ashore as red sand. The dunes were once used as cattle pasture but were abandoned as the early settlers moved inland.
Salt marshes are the second remaining authentic habitat. These bogs are the result of the flooding of low coastal areas during unusually high tides. In the intervals between tides, a marsh area remains and plants take root, notably cord grass, the “marsh hay” used by the early settlers as winter forage for their livestock. Like the dunes, though, the marshes were soon dismissed as wasteland and escaped development.
The word “trampled” can be replaced by ____________.
A.flooded
B.stepped on
C.burned
D.ripped up

 Read the following passage, and mark the letter (A, B, C or D) on your answer sheet to indicate the correct answer to each question. 
 Visitors to Prince Edward Island, Canada, delight in the “unspoiled” scenery – the well-kept farms and peaceful hamlets of the island’s central core and the rougher terrain of the east and west. In reality, the Island ecosystems are almost entirely artificial.
Islanders have been tampering with the natural environment since the eighteenth century and long ago broke down the Island’s natural forest cover to exploit its timber and clear land for agriculture. By 1900, 80 percent of the forest had been cut down and much of what remained had been destroyed by disease. Since then, however, some farmland has been abandoned and had returned to forest through the invasion of opportunist species, notably spruce. Few examples of original climax forest, which consisted mostly broadleaved trees such as maple, birch, and oak, survive today.
Apart from a few stands of native forest, the only authentic habitats on Prince Edward Island are its sand dunes and salt marshes. The dunes are formed from sand washed ashore by waves and then dried and blown by the wind to the land beyond the beach. The sand is prevented from spreading farther by marram grass, a tall, long- rooted species that grows with the dunes and keeps them remarkably stable. Marram grass acts as a windbreak and allows other plants such as beach pea  and bayberry to take hold. On dunes where marram grass is broken down- for instance, where it is trampled- the dunes may spread inland and inundate agricultural lands or split up fishing harbors. The white dunes or the north coast are the most impressive. There are also white dunes on the east and west coasts. Only in the south are there red dunes, created when soft sandstone cliffs crumble into the sea and subsequently wash ashore as red sand. The dunes were once used as cattle pasture but were abandoned as the early settlers moved inland.
Salt marshes are the second remaining authentic habitat. These bogs are the result of the flooding of low coastal areas during unusually high tides. In the intervals between tides, a marsh area remains and plants take root, notably cord grass, the “marsh hay” used by the early settlers as winter forage for their livestock. Like the dunes, though, the marshes were soon dismissed as wasteland and escaped development.
What conclusion can be drawn from the passage about both the sand dunes and salt marshes of Prince Edward Island?
A.They have never been used.
B.They were once used but have long since been abandoned.
C.They have been used continuously since the island was first settled.
D.They were long unused but have recently been exploited.

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