题目

某商场对顾客实行购物优惠活动,规定一次购物付款总额:(1)如果不超过200元,则不给予优惠;(2)如果超过200元但不超过500元,则按标价给予9折优惠;(3)如果超过500元,其500元内的按第(2)条给予优惠,超过500元的部分给予7折优惠.某人两次去购物,分别付款168元和423元,假设他去一次购买上述同样的商品,则应付款是(   )A.413.7元                    B.513.7元                    C.548.7元                    D.546.6元The Education Department says that school safety has set off alarm bells with frequent reports of serious accidents ________ students got injured or killed. [     ]A. after which  B. in whichC. by which  D. for which