A recent poll of elected officials suggests that candidates, when in the midst of a tough campaign, often make statements about an opponent that they may not think is true.
A. Is
B. are
C. to be
D. of as
E. it is
我觉得这是是一个特殊结构,我忘记叫什么了,大概的意思就是说they may not think是一个插入语,定语从句的修饰词刚好同时是think的宾语时,可以这么使用。只不过这题用了否定的插入语(我甚至不认为是否定的插入语,只是因为they may think is not true不符合think的一般用法,才把not提前的,只是形式上看起来像否定的插入语,但其实上插入的依旧是they may think,只不过not提前了变成they may not think)。
你把定语从句的that放回原位:they may not think that (statements) are true.
其中they may think是插入语,拿出来不影响句子意思:that (statements) are not true.
正是因为they may think是一个插入语,C才不正确的。