At the end of every chapter in our science text, there is a group of optional questions, intended for students who wish to go more deeply into the subject.在我们的科学教科书每一章的结尾，都有一些可自由选择回答的问题。这些问题针对的是愿意进一步深入了解这门课的学生。

orbit n.轨道；绕…轨道而行

Our planet is in orbit around the sun.地球环绕太阳运行。

The earth orbits the sun every 365 days.地球每365天环绕太阳一周。

orchestra n.管弦乐队

Orchestras play symphonies and the music for operas and dance.演奏交响乐和歌剧与舞蹈中的音乐。

orgamsm n.生物体，有机体；微生物

The sea is foil of organisms used as food by fish.大海里充满了生物，鱼类以此为食。

origin n.起源，由来；出身，血统

AIDS has its origin in Africa.艾滋病起源于非洲。

original adj.①最初的，原始的②有创意的

This is the original map before the forest was cut down.这是森林被砍伐之前的原始地图。

The students had to write an original short story for class。学生们得为班级写一部原创短篇小说。

otherwise adv./conj.另外，否则

Come early;otherwise you may not get a seat.早点来，否则你可能找不到座位。

outcome n.后果，成果

The newspaper reported the outcome of the Board meeting.报纸报道厂董事会会议的成果。

outlet n.(河流、烟等）出口

A chimney is an outlet for smoke from a fireplace.烟囱是壁炉向外排烟的出口。

outline n.大纲，概要；轮廓 v.画…的轮廓

I’ll describe the plot of the play in an outline.我会以大纲的形式介绍戏剧竺倩节。

The police outlined the shape of the murder victim on the street with chalk.警方用粉笔在街道上画出谋杀案受害者的轮廓。