How long do you think it will be China sends another manned spaceship to the moon?A. when

问题补充：

How long do you think it will be China sends another manned spaceship to the moon?

A. when

B. until

C. that

D. before

答案：

A. when

B. until

C. that

(答案→)D. before

解析：句意：你认为还要有多久中国才能发射另一个载人飞船上月球。这里do you think 是插入语，这句话用了固定句型：It will be+一段时间+before…，要多久才…，since也可以和一段时间连用，但是是：It is/has been+一段时间+since，所以选D。