In the summer vacation we often take a walk along the river.A might B could C would D shou

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解析：如选A ,只表示与现在事实相反，而主句中would have run 表示与过去事实相反，应该用If I had had time,表示。如果从句中含有系动词、情态动词或助词时，可省略if把从句中的were，had或should提到其主语前形成倒装结构。