Question

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

1. A.
   for
2. B.
   as
3. C.
   though
4. D.
   since

Answer 1

A
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。