It was only after she had read the papers Mary realized the homework before her was extremely difficult to plete.
A. when
B. that
C. which
D. what
网友回答
A. when
(答案→)B. that
C. which
D. what
解析：句意：就是在玛丽读了文章之后，她才意识到她面前的家庭作业非常的难完成。这个句子中含有It was，首先要考虑它是不是一个强调句，把It was和横线去掉，这个句子仍然结构完整，语义清楚，所以说这个句子就是一个强调句，选B。