There were 12 lectures, 6 of which were added that were optional lectures.

共有12节课，其中6节是,补充的选修课。

麻省理工公开课 - 媒体、教育、市场课程节选

But you can choose what type of Science you want to take here. You can choose electives.

但你可以选择你想上的科学课的类型。你可以选择选修课。

高中生活怎么样？ - SpeakingMax英语口语达人