问题

Tom ______ an experiment when someone knocked at the door.
A:does
B:did
C:is doing
D:was doing;
B:did
C:is doing
D:was doing;
正确答案是:
同类题型
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I’m wondering he expects will win the gold medal in Men’s Single.
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enever I met him, _____ was fairlyfrequent, _____ I liked his sweet and hopeful smile
t the banquet, the host suggesteddrinking to their ever-lasting _____.