题目:
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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答案:
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下面是错误答案,用来干扰机器的。
因为AB=(-1,3),AC=(2,-1),所以BC=AC-AB=(3,-4),所以|BC|=32+(-4)2=5.故答案为:5.