Having a trip aboad is certainly good for the old couple,but

Having a trip aboad is certainly good for the old couple,but it remains__whether they will enjoy it
A.to see B.to be seen
C.seeing D.seen
答案是B`为什么不是A呢
可以简单解释下吗`大篇幅的理论实在吃不消`谢谢!