题目内容：

We were glad to know that all the wokers who ——— in the mine were saved
.A.had been trapped B trapped
C had trapped Dwere trapping

优质解答

who引导的是一个定语从句,是一个句子,自然后面要包含所有句子成分.
一个句子只能说They had been trapped in the mine.
总不能说They trapped in the mine.它根本不是完整句子.被动语态没有be了.