帮我解答一题把.各位高手.

39.Jetties, piers designed to aid in marine navigation, are constructed primary of wood, stone, concrete, or combinations of these materials.

各位高手我想问问为和不选C.C 不是要改为复数吗?

B很明显的要改为adv修饰V

stone表示一个种类，不是单个的物

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