The boy is to look after his sister.A. enough oldB. enough young　C. young enoughD. old en

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A. enough old
B. enough young　
C. young enough
(答案→)D. old enough
解析：enough的意思是足够的，可做形容词和副词，一般放在被修饰词的后面。Old年老的；young年轻的。句意：这个男孩年龄足够大照顾他的妹妹了。故选D。