选哪个,请讲下原因,the first time B.at the first time C.for the first time D.at firstPeople are puzzled_____they read the book.

选哪个,请讲下原因,the first time B.at the first time C.for the first time D.at first
People are puzzled_____they read the book.

选择A,.the first time 可单独引导状语从句