“It___Mary who is in the office."
"It___her.I saw her off at the airport just now."
A,can't be```mustn't be           B.must be```can't be
为什么选B不选A
Mr Smith spent the week in the country as he___to work on Sunday.
A.mustn't go    B.didn't have to go
为什么选B不选A
Since she is angry,we___.
A.might as well leave her alone
为什么选A
Why___you always interrupt me?
A.must   B.will
为什么选A不选B
How ___so?
A.dare you say    B.dare you to say
为什么选A不选B
He____the dictionary,so he bought a pencil-box.
A.didn't need to buy    B.didn't need buy
为什么选B不选A

１．must be的否定式为can't be,一般没有mustn't be
２．两个选项都有＂必须＂的意思，但must为主观上认为必要，而 have to则是客观上的必要，上班当然属于后者
３．选项Ｂ上哪儿去了？
４．must常有对某种行为厌烦透顶的意思，翻译为＂你为什么老是怎样怎样．．．＂
５．dare有两种词性，当作为情态动词时，用法与can一样（否定为dare　not，疑问为dare提前），而作为实意动词时，否定为don't　dare　to　do，疑问为do　（you）　dare　to
６．need词性用法与dare相似，这题应该是Ａ

第三题我看不太懂，所以想问问为什么选这个答案？

Mr Smith spent the week in the country as he___to work on Sunday.
A.mustn't go B.didn't have to go
为什么选B不选A
   mustn't go 的意思是"不许去,不可以去"
   didn't have to go 的意思是 "不必去" , 相当于 needn't go.
  上句的意思是"史密斯在乡下度过了那个星期,因为星期天他不必去上班."