Is Mr. Smith in the office?- Yes he is in charge of the office he must be there.A.

问题补充：

--- Is Mr. Smith in the office?

--- Yes， he is in charge of the office，he must be there.

A. however

B. since

C. whether

D. for

答案：

A. however

(答案→)B. since

C. whether

D. for

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