帮我解几道英语单选题

帮我解几道英语单选题
1.-----Do you have to take care of your mother at home?
----Yes.I______
A.do B.have to
为什么不是A,而是B?
2.I shall______help him as much as possible.
A.must B.have to
答案为什么是B?我感觉A也对.
3.When a friend gave Jim a ticket to the game,he_______go.
A.just have to B.can not but C.couldn't help D.couldn't help out
答案为什么是D?
4.------Professor Smith,some students want to see you.
______they wait outside or in your office?
----Show them into my office and tell them I'm coming.
A.Shall B.Do
为什么是A,而不是B呢?
5.———Wasthe test difficult?
------Not at all,we found________.
A.very easy to do it B.it very easy to do
为什么是A却不是B呢?it不应该是作形式宾语的吗?.