问题描述：
A ballet dancer must____for a long period of time before she can sense her own center and balance herself.
A.trained B.have trained C.be trained D.be training
希望能把ABCD都分析下 并解释下句子的意思
A.trained B.have trained C.be trained D.be training
希望能把ABCD都分析下 并解释下句子的意思
问题解答：
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