A terrible earthquake happened in that district at the end of 2004, ________ many countries in the world paid close attention to this．
A.whereB.whenC.whichD.what在线课程B
解析：
本题主要是对非限制性定语从句中关系词的考查。分析句子可知，关系词在此作定语，故排除选项D；根据句子结构，先行词在从句中作状语，从而排除选项C；根据句子的意思可知，先行词应该是at the end of 2004，而不是in that district，故本题的答案为B项。