单选题 ______you understand this rule, you'll have no further difficulty.
  • [A] Once
  • [B] Unless
  • [C] Only
  • [D] Until


【正确答案】 A
【答案解析】[解析] 本题考查连词的用法。once意为“一旦…”；unless意为“如果不；除非”；only意为“只有”；until意为“直到…才…”。