the reason why和the reason that的区别

问题描述:

the reason why和the reason that的区别
The reason ______he didn't come was ______he was ill.
  A.why; that B .that;why C.for that;that D.for which;what
为什么答案是A?

第一空:why在从句中充当原因状语 (如果that就是充当宾语,但后面定语从句中明显不缺宾语,故选why)
第二空:that在后面是引导表语