第一句：do yoi think the reason -which- he gave is believeable.

第一句：do yoi think the reason _which_ he gave is believeable.
第二句：i don not believe the reason why/for which he was late for school.
为什么第一句用which,第二句用Why或for which,the reason 在句中分别作什么成分?请分析它们各自句子成分,谢谢!