because 引导表语从句.并且它的主语只能是it /this/that.
用A the reason语意不通.那就是他过度工作的理由.显然不合适.应该改为他过度工作是他身体垮下来的原因.
用B that必需用the reason做主语.he broke down at last ,the reason is that he had overworked.
用why 表示,的原因.也不合语意.那就是他过度工作的原因.应该改为那就是他倒下的原因.即 that he had overworked was why he broke down at last.