It was with great joy he worked out the problem that he had long been puzzled about.A. wh

问题补充：

It was with great joy he worked out the problem that he had long been puzzled about.

A. which

B. that

C. because

D. since

答案：

A. which

(答案→)B. that

C. because

D. since

解析：题目中含有It was，出现这样的结构，首先要考虑它是不是强调句。如果把这种句型结构划掉后，应该是一个完整无缺的句子；这也是判断强调句型与其它从句的方法。假设它是强调句，去掉it was和空格处，剩下的句子经过调整语序后变成He worked out the problem that he had long been puzzled about with great joy。是一个完整的句子，所以是强调句型。强调句的结构It is/was+被强调部分+ that + 句子的其他成分，如果被强调的部分是人，可以用who代替that。