Mary couldn't open the front door because she _______ the key.
A. had forgotten
B. was forgetting
C. has forgotten
D. forgot
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They told us that they _________ for more than two hours.
A. had been waiting
B. were waiting
C. have waited
D. has waited
Did you do what l _________ you to do yesterday?
A. have told
B. had told
C. had been telling
D. tells
We were hurrying because we thought the bell _________ .
A. had already rang
B. has already rang
C. had already rung
D. rang
从混杂细菌标本中分离病原菌用( )
A. 革兰氏染色镜检
B. 平板划线培养法
C. 半固体穿刺接种
D. 用暗视野显微镜观察
E. 纯培养法