单选题 (Ever since) the world (began) nations (have difficulty) in (keeping) peace with their neighbors.
  • A. Ever since
  • B. began
  • C. have difficulty
  • D. keeping


【正确答案】 C
【答案解析】应改为have had difficulty。因为ever since引导句子的时候，从句用过去时，主句要用现在完成时。