Mark has come in his raincoat and boots, for rain.A. preparingB. preparedC. to prepareD.

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A. preparing
(答案→)B. prepared
C. to prepare
D. prepares
解析： 本句中考察的动词prepare的过去分词转换的固定搭配be prepared for..为…做好准备；如果选择A项，后面就不应该有介词for的搭配，因为prepare是一个及物动词。句义：他穿着雨衣和靴子进来了，已经为下雨做好了准备。故B正确。