这不是考查强调句吗

这不是考查强调句吗　　
 　　
请看下面一题，它是考查强调句式吗？　　
It might have been John __________ bought a new book for Mary yesterday. 　　http://yingyu.chazidian.com
A. what                        B. since                        C. that                        D. then 　　
此题应选 C。此题的难点在于考生看不出这是一个强调句型，因为主句的谓语动词be采用了might have been这一复杂形式。此句若改为非强调句，则可以是：　　
John might have bought a new book for Mary yesterday. 约翰昨天可能给玛丽买了一本新书。　　
将此句改为强调句(强调主语)，则可以有两种形式：　　
a. It is John that might have bought a new book for Mary yesterday. 　　
b. It might have been John that bought a new book for Mary yesterday. 　　
又如：　　
It must be a girl that wrote the letter. 写这封信的一定是个女孩。　　
It might be yesterday that he left for Beijing. 他可能是昨天去北京的。　　
Can it be Jim that has stolen the money? 偷钱的会是吉姆吗?　　
It may be next week that he leaves for Paris. 他可能是下个星期去巴黎。　　
It might have been her boyfriend that bought so nice a handbag for her. 给她买那样好看一个手提包的可能是她的男朋友。