问题

Not until all the fish died in the river _____ how serious the problem of water pollution was. [ ]

A: did they realize
B: they did realize
C: had they realized
D: they had realized

正确答案是：

同类题型

Excuse me, can I visit you this afternoon— Maybe you can. Let me check .

_______ well you can drive, you must drive carefully.

When Peggy turned 40, she invited 40 friends to help her _____. [ ]

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We prefer at the party rather than_____.