1.please don't stand up until you（ ）
a.are told b.are told to
2.mille was watching tv（ ）andy opened the door
a.as b.when
3.he was seen （ ）into the room with a knife in his hand
a.to go b.go c.to going d.gone

1。选B。please don't stand up until you are told to在这里省略了一个stand up.
全句补全的话；Please don't stand up until you are told to stand up， 是不是太绕舌了，于是把与前面重复的部分（stand up）省去了。

2。选B。Mille was watching TV when Andy opened the door.

"Andy开门的时候Mille正在看电视。

as与when的区别：

as 表示的是“在。。。的时候”，“同时”不过在as的前后应该是同一个时态：
例：I slipped on the ice as I ran home.
我跑回家时在冰上滑了一跤

一个英英词典里对于as的解释：

conjunction

1。in the same amount in which or to the same degree to which： 例it flew straight as an arrow
2。in the same manner that; 例：according to the way that do as you are told
3。at the same time that;例： while she laughed as she spoke
4.because; 例：since as you object, we won't go
5.that the consequence is；例： the question is so obvious as to need no reply
6.though；例：tall as he was, he couldn't reach it
7.Informal that； 例： I don't know as I should

看这里面的第三种解释，再看看例句，想你应该明白了。

when “当。。。的时候”，一边是过去时，一边可以用进行时。
应该不需要举例了吧。

3。选A。

see sb. do/doing

在变为被动语态时，要加上to。那么，这个句式可以有两种改法：

be seen to do sth.

be seen doing

see 后面可以接不加to的不定式，也可以接动词的ing形式。

所以选A。如果C改为going（去掉to），也可选。