He was much more thorough in his studies than the rest of his cl

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】：译文：他在学习方面比班里其他同学都认真细心，正因为这个原因他才取得了如此大的进步。此句是一个由and连接的并列句。后一个分句中，句子的各个成分都完整，for this reason为原因状语，所以这并不是一个由why引导的定语从句而是It is...that强调结构。答案为C。