He left a few thousand troops to guard Richmond, then took the rest north.

VOA: special.2009.09.24

OCW Self learners are coming to OCW to explore whole new areas of study as well as to brush up on subjects that they took a while ago and perhaps got the rest in the end.

自学者浏览,搜索全新领域的研究,补充学过的科目的知识,使之完整。

麻省理工公开课 - 媒体、教育、市场课程节选