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
网友回答
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。