英语问题 ----are the days when teachers were looked down upon. a

英语问题 ____are the days when teachers were looked down upon. a.gone b.go c.to go d.going 选A,WHY
____are the days when teachers were looked down upon.
a.gone b.go
c.to go d.going
选A,为什么,并解释这句话的含义