秋风泡泡
关键是看the reason在定语从句中作什么句子成分，如果做主语，宾语，则应该用that.如果作状语，则应该用why.例如： He didn't explain the reason why he was late this morning. 这里the reason在从句中作状语，表示原因。 The reason that he gave for his lateness was that he didn't catch the first bus.
我叫鑫小鑫
1.C这是他们都反对这个计划的原因。2. C这是他要走的原因。两句没什么区别，why引导的都是定语从句，修饰名词reason. why在定语从句中表示原因。比较：The reason why they are all against the plan is that the plan is not good.他们都反对这个计划的原因是这个计划不好。The reason why he is leaving now is that you are too rude.他要走的原因是你太粗鲁。上面两句中，that从句说的就是reason本身，是reason的同位语从句。
晨阳爱美食
区别如下：1、前者有 why 是关系副词，后者的 that 是关系代词。2、前者表示从句动作发出的理由，后者表示该原因仅局限于整个从句的内容范围。The reason why he came back is unknown.The reason that he came back is that he needs some rest．
