查了一下词条,both作pron.的话,意思为the one as well as the other,显然这里如果用both (of the contractor and the engineer)会和 the other有冲突。将解释带入原句: the contractor and the engineer blame the other .(as well as = and ),显然用both不合适,因为the other 一般会用在 one ..., the other...中;所以A不对.