——it is to jump into a river in summer a how fun b how funny c what a great fun d what great fun

为什么选d呀，能给我全部详细解释一下吗

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因为你把它换成正常语序就是
It is a great fun to jump into a pool or go swimming in a river in summer
a不行是因慧盯链为fun是名词 how 后一定跟形容词
不过b 我觉得说得通 倒装句，强调很有趣，所以把How funny放则烂前面，属于How +表语+主+系动前孙词结构 it照样是形式主语，而主语是to jump into a river in summer

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