咖啡因的作用是（）。

A.兴奋大脑皮层

B.抑制胃酸分泌

C.收缩支气管

D.舒张脑血管

The achievements of the greatest minds in science could never have been reached if it had not been for the patient and accurate work of hundreds of other people.

A．has it not been

B．if it had been

C．if hasn't been

D．had it not been