The problem was so complicated that Mr. Smith had to______his brains.A.crackB.checkC.rackD

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The problem was so complicated that Mr. Smith had to______his brains.

A.crack

B.check

C.rack

D.block

请帮忙给出正确答案和分析,谢谢!

热搜:初始t细胞是指从未接受过抗原刺激的尚未成熟的t细胞