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某情报站有A,B,C,D四种互不相同的密码,每周使用其中的一种密码,且每周都是从上周未使用的三种密码中等可能地随机选用一种.设第1周使用A种密码,那么第7周也使用A种密码的概率是____.(用最简分数表示)
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     The World Meat Free Day for 2017 falls on 12, June, but what would actually happen if the whole world suddenly went vegetarian(素食的) permanently?

     If vegetarianism were accepted by everyone by 2050, the world would have about seven million fewer deaths every year. No meat would lower the chances of some diseases or even some cancers and so save the world 2-3% global GDP in medical bills. Food-related emissions would drop by around 60%. This would be due to getting rid of red meat which come from methane (甲烷) producing livestock (家畜) — from people's diets. Turning former pastures (牧场) into native habitats and forests would slow climate change and bring back lost biodiversity, including some animals which were previously pushed out or killed in order to keep cattle.

    However, farmers in the developing world could really suffer. Arid and semi-arid lands can only be used to raise animals as there's no or little water in these places, such as the Sahel land strip in Africa next to the Sahara; nomadic (游牧的) groups that keep livestock there would be forced to settle permanently and lose their cultural identities if there were no more meat. Losing meat also means that we also lose traditions. There's no more Christmas turkey. We would need to replace meat with nutritional substitutes, in particular for the world's estimated two billion-plus unhealthy or weak people for lack of nutrients. Animal products contain more nutrients per calorie than staples like grains and rice.

    Everybody currently engaged in the livestock industry would need to be retrained for a new career. This could be in agriculture, reforestation or producing bioenergy. Failing to provide career alternatives could lead to mass unemployment and social problems, particularly in traditionally rural communities.