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

引起的运动性腹痛原因准备活动不充分开始运动时运动量过大，内脏器官的功能还没有提高到应有的运动水平就加大运动强度等，都可以引起（）。