Taking a trip abroad is certainly good for old couples but it remains whether they will

问题补充：

Taking a trip abroad is certainly good for old couples, but it remains whether they will enjoy it.

A. to see

B. to be seeing

C. to have seen

D. to be seen

答案：

A. to see

B. to be seeing

C. to have seen

(答案→)D. to be seen

解析：句意：去国外旅行对这对老夫妻是好的，但是否他们喜欢还有待以后再看。这里it是形式主语，whether they will enjoy it是真正的主语从句，这里it与see之间是被动关系，故选D。