单选题 We don't know the reason ______ he didn't show up.
  • A. that
  • B. which
  • C. /
  • D. why
【正确答案】 D
【答案解析】[解析] 该句为限制性定语从句，先行词the reason在定语从句中充当原因状语，故使用关系副词why引导。