The teacher asks us not to stand up until _____ in the Engli

The teacher asks us not to stand up until _____ in the English class.
A. told B. being told C. told to D. having told
为什么选 C

the teacher asks us not to stand up until (we are) told to (stand up) in the English class.
=the teacher asks us that we should not stand up until we are told to in the english class.
我下面=后面写的这句是宾语从句中套有一个状语从句：在状语从句中,如果状语从句的主语和主句的主语相同,而且状语从句中又包含be动词,那么状语从句中的主语和be动词可以一起省略：
in English class,we should not stand up until （we are） told to stand up.
we和 tell之间是被动的关系,
now understand?
再问： 谢谢呀