* compel
v. 强迫，逼迫
The law will compel employers to provide health insurance.
这项法律将强迫雇主提供健康保险。
She felt compelled to resign because of the scandal.
因为这个丑闻，她感到必须辞职。
* compulsion
n. 强烈的欲望，冲动
Leith felt an overwhelming compulsion to tell him the truth.
利思感到有一股强烈的欲望要把实情告诉他。
* compulsory
a. 强制的，强迫的
It is compulsory for all motorcyclists to wear helmets.
所有骑摩托车的人都必须戴头盔，这是强制性的。
English is a compulsory subject at this level.
英语在这一级次是必修科目。
* optional
a. 可选择的

* expel
v. 开除
Two girls were expelled from school for taking drugs.
两个女生因吸毒而被学校开除。
The main opposition leader was expelled from her party.
主要反对党的领导人被开除出党。
* propel
v. 推动，推进，驱动
a boat propelled by oars
用桨划的船
* propeller
n. 推进器，螺旋桨
* repel
v. 排斥
Two positive charges repel each other.
两个正电荷相互排斥。
a lotion that repels mosquitoes
驱蚊液
The army was ready to repel an attack.
军队已准备好击退进攻。
The smell repelled him.
这气味让他反感。