—Last night I watched the football match and didn't go to bed___12 o'clock.

—Last night I watched the football match and didn't go to bed___12 o'clock.
—Last night I watched the football match and didn’t go to bed＿＿＿＿12 o’clock．（2002黄冈市）
A． when B． until C． as D． while为什么要选B而不是A呢?求解析,谢谢