1.It isn't cold enough for there____ a frost tonight,so i can leave his car out quite safely.

1.It isn't cold enough for there____ a frost tonight,so i can leave his car out quite safely.
A.would be B.being C.was D.to be
2.The number of registered participants in this year's marathon was half________.
A.of last year's B.those of last year's
C.of those of last year's D.that of last year's
3.I would have gone to visit him in the hospital had it been at all possible,but I ____fully occupied the whole of last week.
A.were B.had been C.have been D.was
(此题为何不能选B?)
第三题是选B，为什么不能选D？