问题补充：
. Will you please keep quiet now? You shouldn’t talk until to.
A. asking
B. having asked
C. to ask
D. asked
答案：
A. asking
B. having asked
C. to ask
(答案→)D. asked
解析：考查状语从句的省略。句意：请你现在保持安静好吗，你不应该说话直到你要求被说。完整形式为：You shouldn’t talk until you are asked to.故选D。
