网络主干上的四台交换机A、B、C、D的优先级分别为8192、4096、12288、16384，那么这四台交换机中可能会成为根交换机的是______。
A) 交换机A
B) 交换机B
C) 交换机C
D) 交换机D
参考答案：B
解析： 在根路由器的选择中，优先级最高的交换机将当选。优先级的值越小，则优先级越高，本题中交换机B的优先级最高。
World Wildlife Fund (WWF)'s goal is to save the world's environment. The organization was set up ___21__September 11，1961. In the beginning, WWF worked __22__ the world's wildlife. Now, it works to save Earth__23___ its environment. WWF has many tasks. One is to stop air pollution. Pollution from industry(工业) and cars is _24_ to our environment. Each year, more and more trees are cut down __25__WWF tries to protect Earth's forests．Also people use more land, more and more animals are __26__ their homes and many die. WWF can buy important areas to __27__ the land from disappearing because of human development. WWF has completed many projects. In China, WWF works to save pandas from extinction（灭绝）． ___28__South America，it works to save the Amazon Rainforest. Volunteers and donations help WWF complete its work. WWF's work is __29__to our environment, and is also important for people to help them __30__ time and money.
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