【答案解析】must have rained“一定下了雨”。连词for引导的从句用来解释原因，为这一推断提供证据。must后面跟完成式表示说话者以肯定的语气推断过去某个时候或在说话前不久已经完成的动作“一定发生了某事”，如：He must have arrived by air。如果要推断过去某时或在说话前不久一定正在发生某种情况需在must后面用完成进行时must have been-ing，如：We must have been playing tennis in the park when you phoned．must+行为动词表示“一定要，必须”；must be(或跟状态动词)表示“一定是，准是，想必”，如：Judging by thesmell，the food must be delicious．might have rained“也许下了雨”，“本来可能会下雨的(但实际并未下雨)”。should have rained“本应下雨(但实际并未下雨)”。