I had not been reading for half an hour _____ I heard steps outside.

A. while
B. as
C. when
D. since
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科目：高中英語 來源：0125 模擬題 題型：單選題

I had not been reading for half an hour _____ I heard steps outside.
[     ]
A. while
B. as
C. when
D. since

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