下列哪种情况需要启动ARP请求（）
A.主机需要接收信息，但ARP表中没有源IP地址与MAC地址的映射关系
B.主机需要接收信息，但ARP表中已经具有了源IP地址与MAC地址的映射关系
C.主机需要发送信息，但ARP表中没有目的IP地址与MAC地址的映射关系
D.主机需要发送信息，但ARP表中已经具有了目的IP地址与MAC地址的映射关系
参考答案：C
| 阅读短文还原句子。 | |
First, there is the school or work friend. This is the person who helps you get through the school or work day. But you never really go out with them outside these places. However, they are important in your life as you will spend a lot of time with them. 2 _____.You can't really discuss anything important with them. But if you are feeling sad, they're great to go out with because they can cheer you up and make you for- get all your troubles. The next kind of friend is really unusual and having one makes you very lucky. They are really kind so they pay for you every time you go out. They usually have many of the things you can't afford to buy. 3_____. Finally, there is the best friend. This is the person who knows you inside out. The person you would have no problem spending the whole day with. Friends are the most important thing in your life. 4 _____. | |