---where is the comic book?---i brought it to you -----you were in the reading room yesterday.when if because before 选哪个?
问题描述：
---where is the comic book?---i brought it to you -----you were in the reading room yesterday.
when if because before 选哪个?
答
when
显然发问者不知书在哪儿，由此可知第二人送书时他不在，同时可知这是一个时间状语从句，只能选when
答
选when.when引导的是时间状语从句，
答
when