问题

This kind of game is exciting.—Yes, but it’s dangerous. One can’t do it ________ he or she has got some training.

A:if
B:after
C:when
D:Unless;

正确答案是：

同类题型

l the truth, it’s the that makes my job so enjoyable.

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ir marketing plans succeed, they ____their sales by 20 percent.

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