"I go to bed at about 11 o'clock.句中为什么用"about"?

"I go to bed at about 11 o'clock.句中为什么用"about"? I go to bed ( ) about nine o'clock every day.A.at B.in C.on It's about 11 o'clock.Kate must go to bed (改为同义句） I often go to bed at（）11 o'clock Go to bed at nine o'clock.改为否定句 He go to bed at ten o 'clock.(否定句) I go to bed at nine o'clock. i go to bed at nine o'clock every evening对 nine o'clock 提问 go ten the bed about at in I thirty to evening 连词成句 I must go to bed by 11:00 o' clock 同义句转换 I went to bed at 11 o'clock last night的同义句 I get into bed at about 11:30 and go right off to sleep.