题目内容

当一个帧离开路由器接口时,其第二层封装信息中()。

A. 数据速率由l0BASE-TX变为100BASE-TX
B. 源和目标IP地址改变
C. 源和目标MAC地址改变
D. 模拟线路变为数字线路

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_____ it rain tomorrow, we would have to put off the visit to the Shanghai Museum.

A. If
B. Should
C. Were
D. Would

_____ your letter, I would have written back two days ago.

A. If I received
B. Should I receive
C. Had I received
D. If I could have received

If Mother _____ me that truth when I _____ young, I would not make such mistakes so often now.

A. has told; was
B. told; was
C. told; had been
D. had told; was

If it were not for the fact that you _____ ill, I would ask you to do this right now.

A. were
B. had been
C. are
D. should be

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