为何答案选time enough

为何答案选time enough
__________,they will go and visit the zoo.
A Time enough B Time is enough C If time willbe enough
D With time enough
我知道答案为何不选B和C,但不了解,为何不能选D呢?
不选c的理由是If引导的条件状从表将来时态时，从句用一般现在时，主句用将来时。因此C应该改为If time is enough才行。