题目

Thomas Alva Edison didn’t “invent” the light bulb (灯泡), but rather he ______ a 50 –year –old idea, which is not well –known. A. improved upon       B. take down       C. put up        D. break away 答案：A 解析: 句意：鲜为人知是的，爱迪生并没有发明灯炮，而是在其50年前的发明上改进了电灯泡（把灯丝寿命延长到13.5小时）。Improve on/upon超过，改进；take down把……拆卸开；put up张贴，搭建；break away摆脱。若O为△ABC的外心，且∠BOC=60°，则∠BAC=    ．