Henry must have stayed up last night he looks so tired and sleepy.A. forB. asC. thoughD.

问题补充：

Henry must have stayed up last night, he looks so tired and sleepy.

A. for

B. as

C. though

D. since

答案：

(答案→)A. for

B. as

C. though

D. since

解析：考察连词。A.因为 B.因为；随着 C. 尽管 D. 因为；既然。for表示所说的理由是一种补充说明, since和as是从属连词，说明的是已为人们所知和理由。句意：亨利昨晚肯定熬夜了。因为他看起来如此累和困乏。结合语境可知选A。