单选题 Many agents quit and say they are leaving because they are ______ and want better pay and more humane working conditions, including less travel.
A. reverted to B. backed up C. fed up D. thrust in

【正确答案】 D
【答案解析】[解析] revert to“回复，归还”；back up“支持，倒退”；feed up“供给……食物”；thrust in“突然提出，不恰当地插入”。