定语从句中介词+关系代词结构 — 英语填空题
题目
合并句子，并在空格处填入“介词+关系代词”结构；保留短语 refer to：The safety plan listed three rules. The inspectors referred to the third rule before approving the plan. → The third rule, ___ the inspectors referred before approving the plan, was especially important.