单选题 ______ arriving at the burning building, the fire fighters at once joined the local residents in the fight against-the big fire.
  • A. On
  • B. For
  • C. At
  • D. In
【正确答案】 A
【答案解析】[解析] 介词 本题考查介词。n的用法。on doing sth.表示“刚一．．．”，为固定用法。