问题
The world is changing fastI strongly believe that one will easily fall behind _______ he keeps on learning
A:unless
B:if
C:when
D:as
B:if
C:when
D:as
正确答案是：
同类题型
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So excited was the winner that he couldn’t say anyword to his feelings.
For a moment nothing happenedThen all shouting together[ ]
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