局域网交换机具有很多特点。下面关于局域网交换机的论述中说法不正确的是________。
A. 低传输延迟
B. 高传输带宽
C. 可以根据用户级别设置访问权限
D. 允许不同传输速率的网卡共存于同一个网络
The vocabulary and grammatical difference between British and American English are so trivial and few as hardly ________.
A. noticed
B. to be noticed
C. being noticed
D. to notice
如果用户应用程序使用UDP协议进行数据传输，那么下面________必须承担可靠性方面的全部工作。
A. 数据链路层程序
B. 互联网层程序
C. 传输层程序
D. 用户应用程序
He ________ in the next room.
A. is often heard to sing
B. often heard to sing
C. is often heard sing
D. is often hearing to sing
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