—You look so tired. What’s the matter?—I was so busy that I didn’t go to bed 12 o’clock la

问题补充：

—You look so tired. What’s the matter?

—I was so busy that I didn’t go to bed 12 o’clock last night.

A. until

B. unless

C. if

D. so

答案：

(答案→)A. until

B. unless

C. if

D. so

解析：until直到；unless除非，如果不；if是否，如果；so因此。句意：你看起来是这么累。怎么了？——我是如此忙，以至于我昨晚直到十二点才睡觉。短语not……until直到……才，结合语境可知选A。