按照用户界面的使用环境和功能特征的不同,一般可以把操作系统分为三种基本类型,即:( ),( )和实时系统
No more having to buy expensive textbooks which you will only use for a year or two and then sell or _______ away.
You can _____ some bottles of wine, or some chocolates, or a bunch of flowers.
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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