题目内容：

Grammar语法
All the neighbors admire this family ____ the parents are treating their child like a friend.
A.why B.where C.which D.that
____ you eat the correct foods ____ be able to keep fit and healthy.
A.Only if;will you B.Only if;you will
C.Unless;will you D.Unless;you will
-Can I smoke here?
-Sorry.We don't allow____here.
A.people smoking B.people smoke C.to smoke D.smoking
为什么不是A,smoking可以做宾补啊.答案却是D.
请根据我的提问作答,并且解释.
谢``````