一道英语选择题不懂

问题描述:

一道英语选择题不懂
We cannot accurately predict how much carbon dioxide _____by factories will remain permanently in the air.
答案是released.为什么不是being released?不是被动语态吗?水讲得详细分给谁

be released == Vpp 是被动语态
which is/was 已经省略