单选题 In the Americans' minds, there are boundaries that other people are simply not supposed to cross. When the boundaries are crossed, Americans will______stiffen and their mariner will become cool.
【正确答案】 A
【答案解析】解析：句子的大意为：在美国人的心目中，也有其他人根本不能越雷池半步的界限。一旦有人越过界限，美国人将会变得明显冷酷无情。A项visibly“明显”；B项tragically“悲惨地”；C项uneasily“不安地”；D项tremendously“大大地”。A最合理，所以A项符合题意。