And London's Transport Authority has promoted the Oyster card, a magnetic payment system that keeps track of where people have been and when.

VOA: standard.2010.04.01

So the special active file keeps track of whatever you missed.

因此这些特殊的文件保持着它的轨迹。

哈佛公开课 - 计算机科学课程节选

The World Health Organization keeps track of what countries have cases of polio and when they occur, and what the frequency of--So, this is a map from a few years ago and there are efforts that occur occasionally.

世界卫生组织一直跟踪记录,发生小儿麻痹症的国家及发病时间,以及发生的频率,那么,这是从几年前开始统计的,一个地图,这里是一些偶然产生的成就

耶鲁公开课 - 生物医学工程探索课程节选

So, these memory cells are a way that your immune system keeps track of antigens that it's been exposed to for even if maybe the plasma cells that were producing antibody in response to the initial exposure have died and disappeared.

所以,这些记忆细胞,是人体免疫系统记忆抗原的一种方法,即使由浆细胞产生的抗体,在初次免疫应答中全部被消灭,记忆细胞仍然能迅速分化为抗体分泌细胞

耶鲁公开课 - 生物医学工程探索课程节选