英语问题!十万火急!请给我仔细讲解一下下面这道题：（1）-How many students are there in

英语问题!十万火急!
请给我仔细讲解一下下面这道题：
（1）-How many students are there in the classroom?
- ( ). They are all on the playground.
A.No one B. None C.Nobody D. Everyone
（为什么不选“A”）
（2）People ( ) the southwest of China were in great need of water a couple of months ago.
A. in B. at C. to D. on
（为什么用in,而不用on）
（3）It's very kind ( ) you.Thank you for your help.
A. of B.for C. to D. on