The reason for his success is        （）          he worked hard.

题目解析[填写题目解析]： 题目：The reason for his success is （ ） he worked hard. A.why B.that C.because D.for 答案:B 解析：正确答案是B。句子的正确表达应该是"The reason for his success is that he worked hard."，意思是“他成功的原因是他工作努力”。选项B中的"that"引导了定语从句，修饰先行词"reason"，符合句意。