几道英语选择题，求助，要解析1 I have not seen Mary these past few days.I a

几道英语选择题，求助，要解析
1 I have not seen Mary these past few days.I am afraid she____herself for some time.
A is not feeling
B has not been feeling
C had not been feeling
D was not feeling
答案选B，为什么不选D？
2 Engines are to machines ___hearts are yo animals
A as
B that
C what
D which
答案选C，为什么不选B？
3 The mother did not know ___was to blame for the broken glass as it happened while she was out.
A who
B whoever
C how
D what
答案选A，为什么不选B?
4 success是可数名词吗？能不能说sth is a cuccess?
那能不能说sth is a great success?