According to the passage, why do people join groups that practice secondary cooperation?
A.To associate with people who have similar backgrounds.
B.To get rewards themselves.
C.To defeat a common enemy.
D.To experience the satisfaction of cooperation.

The word “cherished” in paragraph 1 is closest in meaning to __________ .
A.agreed on
B.prized
C.set up
D.defined

It can be inferred from the passage that research concerning the distress signals of trees _________ .
A.is no longer sponsored by the government
B.has been unproductive up to now
C.was conducted many years ago
D.is continuing

Which of the following could be considered a cause of the distress signals of trees?
A.lack of water
B.attacks by insects
C.experiments by scientists
D.torn roots

It can be inferred from the passage that the sounds produced by the trees ________ .
A.are the same no matter what type of tree produces them
B.cannot be heard by the unaided human ear
C.fall into the 1-20 kilohertz
D.serve as a form of communication among trees

The word “them” in line 3 refers to _________ .
A.scientists
B.trees
C.vultures
D.insects

____________ (53)
A.finish
B.end
C.edge
D.stop

____________ (51)
A.knowledgeable
B.recognizable
C.ware
D.intelligent

Mark the letter A, B, C, or D on your answer sheet to indicate the sentence that best combines each pair of sentences in the following questions.
Since 1970, the United Nations has been celebrating April 22nd as Earth Day. It attracts millions of people worldwide to join in to promote public awareness of environmental protection.
A.Since 1970, the UNs has been celebrating April 22nd as Earth Day because it attracts millions of people worldwide to join in to promote public awareness of environmental protection.
B.The UNs has been celebrating April 22nd as Earth Day, which millions of people worldwide are attracted to join in it to promote public awareness of environmental protection.
C.Since 1970, the UNs has been celebrating April 22nd as Earth Day, which attracts millions of people worldwide to join in to promote public awareness of environmental protection.
D.Since 1970, the UNs has been celebrating April 22nd as Earth Day so that more people join in to promote public awareness of environmental protection.

____________ (50)
A.students
B.classmate
C.preference
D.colleagues