答案：C

题目分析:考查非谓语动词。由语境可知此处是回答what引导的特殊疑问句，what在句中做主语，所以用动名词losing，完整回答为：I think losing his seat in the election has made Tom feel down.,故答案选C。句意：--你认为什么让汤姆情绪低落呢？--我想是他在选举中的失利造成的。