有一读者问：如何做此选择题：
(1)Please don't turn on the radio until _____.
A. asked to B.asking to C.ask to D. to ask
应选A。因为 asked to 在此相当于 you are asked to。 until asked to 属于“从属连词+过去分词”省略结构，在英语中，尤其在书面语中，常可见到，再如：
(2)We started exchanging some words in French until stopped by the supervisor.
(3)He will come if asked .
(4)Although exhausted by the climb, he continued his journey.
(5)Some people abandoned personal and national dignity when involved in foreign contacts.
(6)Her car got three bumps while parked in London.
短语从属连词亦可用这种省略结构，如：
(7)She regretted it as soon as uttered .
(8)In order to avoid going deep into the forest the boys worked near at hand on any fallen wood no matter how tangled with new growth.
