第一段词汇解析

①boulders = large stones or rocks。

②perturbed = worried。

③underestimated = guessed that difficulties were not as bad as they really were。

第二段词汇解析

①bumped = moved along in an uneven way。

②swerved = moved quickly in one direction, then in another。

③hammered = hit repeatedly。

④ominously = as if giving a warning that something unpleasant is going to happen。

第三段词汇解析

①what a relief it was，是感叹句，一般是 what a/an adj. 单数可数名词 主语 谓语，当然，也可以接复数名词和不可数名词，但是 how 引导则只能接单数名词，不能接复数名词和不可数名词。当单数时可互换， How interesting a story it is!=What an interesting story it is!特别注意语序。

②in response to，作为……的回应。