[10-20 18:10:12] 来源：http://www.67xuexi.com 高考英语复习资料 阅读：85254次
Were you in San Francisco at the time of the big earth quake in 1989﹖
1989年旧金山发生地震时，你在那里吗?
It happened at the time of King Alfred. 事情发生在阿尔弗雷德国王时期。
◇ at that time 则通常指前文明确提到的某个时期、时候。通常其后不带“of...”短语。例如：
In the 17th century much corn was grown in Tibet and Sichuan.At that time
(=At the 17th century) the land along the Changjiang River was becoming very crowded.
◇ at one time=during a period of time in the past意为“过去有一段时期”，“曾经”。例如：
They used to be good friends at one time. 他们曾经是好朋友。
◇ at a time则意为“一次”，表示一个时间单位。它常与表示数量的词语连用，表示频率。例如：
Don’t speak all at once.One at a time, please. 不要同时一起说。一次只一个人说。
Take the medicine three times a day and three pieces at a time. 这些药每天服三次，每次服三粒。
◆ at ... speed / with ... speed
◇ at the speed of或者at ... speed，意为“以……的速度”。而当speed被all, lightning, great等修饰时，介词应用with。我们可用一句口诀来帮助记忆：都(all)以闪电般(lightning)大(great)的速度行驶。如：
Our car was running with all speed on the expressway. 我们的车在高速公路上全速行驶。
The Long March No.2 Rocket sent up the satellite into space at the speed of 11.2 kilometers per second. 长征二号火箭以每秒钟11.2公里的速度将卫星发射到太空。