The boy did his homework with the help of his father in the past. But now he can do it alone.A.for himselfB.on himselfC.by himselfD.with himself
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C解析for himself为自己； on himself靠自己；by himself通过自己，靠自己； with himself和自己。句意：在过去，这个男孩在他的父亲的帮助下做作业，但是现在他能够独自完成了。on himself前与搭配动词depend结合语境可知选Ｃ。
考点：短语辨析