exciting sport it is to climb the mountains！A. HowB. How anC. What aD. What an

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A. How
B. How an
C. What a
(答案→)D. What an
解析：以what引导的感叹句的基本句型结构为what a/an +adj+可数名词单数+主语+谓语！；what+adj+可数名词的复数/不可数名词+主语+谓语！以how引导的感叹句的基本句型结构为how+adj/adv+主语+谓语！