问题补充：
What you at this time yesterday?
A. did ; do
B. is; doing
C. were; doing
D. are ; doing
答案：
A. did ; do
B. is; doing
(答案→)C. were; doing
D. are ; doing
解析：at this time yesterday是过去时的显性标志，强调当时正在做什么。故选C.
翻译为“昨天的这个时间你们正在做什么？”
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