It is not until recently the scientists have found proof of there being water on Mars. A.

问题补充：

It is not until recently the scientists have found proof of there being water on Mars.

A. when

B. that

C. how

D. what

答案：

A. when

(答案→)B. that

C. how

D. what

解析：可联系答.案.网客服索取。