单选题 The fire brigade arrived two hours later after the alarm was sounded, ______ it was too late to save much of the building.

A. then
B. when
C. which
D. though

【正确答案】 B

【答案解析】[解析] 题中要求填进一个具有连接作用的词，将前后两个句子连起来，所以首先排除then；分析前后两个句子之间的关系得知，though也是不对的，因为此处显然不是转折关系；which，when引导非限制性定语从句，故接下来关键是要找出哪个词是先行词，很明显是two hours later，所以应该选when，表示时间“这时”。