Children are to meet with setbacks as they grow up, so their parents don’t have to worry

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A. possible
B. due
C. necessary
(答案→)D. bound
解析：句意：孩子们在长大的过程中，一定会碰到一些挫折，所以他们的父母不要担心。be bound to 注定，一定要，be possible to 可能的 be due to由于 be necessary to是必要的，根据题意，故选D。