什么是缓冲? 为什么要引入缓冲? 操作系统如何实现缓冲技术?
文件目录的作用是什么？文件控制块（FCB）主要包括那些内容？
There are about ____ students in our school.
A. two thousand of
B. two thousand
C. two thousands of
If you want to go to the cinema, take _____.
A. bus No. the ninth
B. the No. 9 bus
C. No. 9 the bus
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