一、单项选择（共15小题；每小题1分, 满分15分）
1. experiments of this kind ________ in both the u.s. and europe long before the second world war.
a. conducted b. was conducted c. had been conducted d. had conducted
2. no scientists 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
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