He was much more thorough in his studies than the rest of his class and it was for this reason（ ）he made such great progress.
A、
why
B、
how
C、
that
D、
for
【正确答案】：C
【题目解析】：
把it was...that去掉，剩下的句子：for this reason he made such great progress.句子完整，所以是强调句。选C. that。句意：在他的研究中，他比他班上其他人更努力，正是出于这个原因，他取得了如此巨大的进步。