1. SOURCES OF ENERGY

At present, most of our energy comes from fossil fuels (coal, oil and natural gas). However, reserves are limited. Because demand for electricity is rising rapidly, fossil fuels will be exhausted in a relatively short time. Therefore, people must develop and use alternative energy sources.

An alternative energy source is nuclear power. Nuclear power can provide enough electricity for the needs of the world for hundreds of years, but it can be very dangerous. An alternative source of geothermal energy comes from deep within the earth. Scientists use heat to generate energy. However, this energy is only available in some parts of the world.

Sun, water and wind are alternative sources of energy. Wind rotating the windmill and moving the sailboat. It's a clean source and a lot of energy. Unfortunately, if the wind does not blow, there is no wind energy. As water moves from one place to another, it makes energy. However, people have to build dams to use this energy source. The dams cost a lot of money, so water is expensive too.

The sun releases large amounts of energy each day. This energy can be converted into electricity. Many countries use solar energy for their daily lives. Solar panels on the roof can generate enough energy to heat or cool the entire home. Solar energy is not only abundant and infinite but also clean and safe.

Despite some drawbacks, the potential of alternative energy sources is enormous. However, how to make full use of the energy resources is a question for researchers all over the world

2. TRAVELLING IN THE FUTURE

I think in the future there will be many modern means of transport such as speed trains, trains, submarines, airplanes, space trains, unmanned vehicles..... But my favorite vehicle is the airplane because it is beautiful and environmentally friendly, especially when it gets too much traffic, it can fly to the sky so that we can move faster and without losing much time. We can also use it to tour the sky. It is the perfect combination between cars and planes. It has a drawback as it is very expensive so not many people use it. If I have a lot of money in the future, I'll buy one to take my family to the sky and more places in the

3.AN OVERCROWDED WORLD

It took hundreds of thousands of years for the world population to grow to 1 billion. However, in just another two centuries, it grew sevenfold. You know, in 2011, the total population of the world reached 7 billion. And today, it's about 7.5 billion. Tokyo, with 37.8 million residents, continues to be the largest city in the world in terms of population.

Advances in modern medicine are the main cause of this dramatic growth in the world population. It helps the birth rates increase and the death rates decline. The second contributor is the improvement in living standards. This leads to the rise of global lifespan, from 64.8 years in the early 1990s to 70.0 years today. Last but not least, the number of women of reproductive age is increasing day by day.

Ok, that is 3 paragraph about 3 unit