The reason why he was late was ________ it rained heavily.     [ ]

A. why

B. that

C. whether

D. because

做膝跳反射实验前先告知受试者有思想准备，结果实验现象不明显。其原因是：（）

A.反射弧结构发生变化

B.传出神经受阻

C.感受器受到抑制

D.高级中枢对低级中枢有调节作用