It was only after he had read the papers Mr. Gross realized the task Before him 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：考查强调句型。only after he had read the papers是被强调的部分，Mr. Gross realized the task Before him was extremely difficult to plete句子成分完整，排除C、D；把It was和that去掉后不影响句意，因此是强调句，故选B。
【考点定位】强调句型it is...that。