I’m fine — I just felt a bit sleepy and tired that was I went to bed so early.A. because

问题补充：

I’m fine — I just felt a bit sleepy and tired, that was I went to bed so early.

A. because

B. when

C. that

D. why

答案：

A. because

B. when

C. that

(答案→)D. why

解析：考察表语从句.句意,我很好——我只感到有点困和累，那是为什么我这么早睡觉的原因。可知前文为下文的原因，故选D。