It's __ eight o'clock,I must go.空格应选哪个介词?为什么?about ,at ,for 等答案是a_英语解答

It's __ eight o'clock,I must go.空格应选哪个介词?为什么?about ,at ,for 等
答案是about哦,可语法上怎么解释不选at呢?

人气：169 ℃　时间：2020-06-04 11:00:50

解答

about 大约,差不多的意思
现在差不多8点了,我得走了