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【判断题】
所有痕量元素，当其浓度超过安全界限时，可能具有毒性。A.
正确
B.
错误
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【单选题】When scientists first warned in the 1970s that CFCs could attack ozone, the U.S. responded by banning their use in spray cans. But the rest of the world continued to use CFC - based aerosol cans, and ...
A.
Because the CFCs directly damage the people’s health.
B.
Because the CFCs are poisonous chemicals.
C.
Because the production of the CFCs costs a lot.
D.
Because the CFCs can attack ozone by liberating atoms of chlorine.
【单选题】Scientists first______the idea of the atomic bomb in the 1930s.
A.
competed
B.
conceived
C.
consumed
D.
concealed
【单选题】Scientists first ______ the idea of the atomic bomb in the 1930s.
A.
perceived
B.
conceived
C.
deceived
D.
retrieved
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