简单叙述分组交换网的组成。
参考答案：分组交换网由分组交换机、网管中心、分组装拆设备和远程集中器、分组终端和传输信道组成。
补全对话。
A:Hi, Jack. How l your vacation?
B:It was pretty good.
A:Where did you _ 2 ?
B:I went to Beijing 3 my parents.
A: 4 did you go there?
B:We 5 _ there by train.
A: 6 did you do?
B:We visited Tian'an Men Square. There 7 _ many people there.
A:How were the people?
B: 8 were very friendly.
A:How was the 9 ?
B: 10 was sunny and cool.
水和H2S同时存在对加氢脱氮（HDN）有明显的促进作用。
