问题
entists can exactly figure out what ________ happened ________ at that time, which caused the extinction of the dinosaurs.
A:on earth; on the earth
B:on the earth; on earth
C:on earth; on earth
D:on the earth; on the earth;
B:on the earth; on earth
C:on earth; on earth
D:on the earth; on the earth;
正确答案是：
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