单选题 Why can"t you do this small ______ for me? I"ve helped you often enough in the past.
【正确答案】 D
【答案解析】[解析] do sb. a favor或者do a favor for sb. 为固定搭配，意为“给某人帮忙”。故选项D为正确答案。command：命令；demand：要求；effort：努力，均不符合句意。