Noscientistscanexactlyfigureoutwhat

問題詳情：

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

【回答】

A

知識點：詞組 短語

題型：選擇題