我这里有几道英语的单项选择题,我不太懂,

1.---Shall we go outing this weekend?
---Oh,with all this work( a),I don't know if I'll have time to go out
A to do要做 B doing正在做 C done已经做了 D do
2.Working in an air-conditioned office,you can hardly imagine what a hard time the peasants have ( b)rice.
A to growing B to grow表目的 C grown D growing
3.( b) the temperature falling so rapidly不是句子,we could't go on with the experiment.
A since B with C as D for 都表原因,如果用acd应该是句子
4.The Whites are( b)holiday .But I don't know where they have gone( )their holidays.
A in;for B on;for C for;in D at; on on holiday度假,固定短语
5.He has three brothers,( )is a doctor.
A all of them B all of whom C none of them D none of whom(这道题的答案是D,为什么C 不行呀?）因为这句话有个逗号,不能用c,如果用c,可以改为He has three brothers,and none of them is a docter