问题补充：
She often goes to work .
A. by a underground
B. by underground
C. by the underground
D. take the underground
答案：
A. by a underground
(答案→)B. by underground
C. by the underground
D. take the underground
解析：句意：她经常坐地铁上班。短语by underground坐地铁，中间不需要冠词；或者可表述为She often takes the ground to work.结合语境故选B。
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