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叉树是结点的有限集合，这个有限集合或者为空集，或者由一个根结点及两棵不相交的、分别称为根的左子树和右子树的______组成。
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The burning of coal is very wasteful of energy. This can be realized when we remember that one pound to coal burned in the furnace of a power station (26) raise enough team to drive a generator (27) will produce enough current to light a one barelectric fire for three hours. On the other hand, if all the energy in the atoms of a pound of coal could be released, (28) should be enough energy to (29) all the machinery in all the factories in Britain for a month. In simpler words, all this means that one pound of any element or compound of element, if completely converted (30) energy by breaking up the atoms, would (31) the same amount of heat (32) the burning of 1,500,000 tons of coal. Scientists have calculated that if a bucket of sand from the beach could be completely (33) into energy, and if the energy so obtained was used to drive electric generators, enough (34) would be produced to supply the whole of Europe for five years. (35) other words, a bucket of sand contains enough energy to (36) a thousand million pounds’ worth of electricity. Albert Einstein was the first man to realize the vast (37) of energy locked in the atom. Einstein was also the first to show in theory how the energy of the atom might be released. Indeed without Einstein’s theory (38) was the result of many complicated mathematical calculations, there would never have been any practical (39) of nuclear power. In 1905, which was many years before other scientists really understood a great deal about atomic energy, Einstein declared that if you "destroyed" a given mass of matter, you would obtain (40) its place an enormous amount of energy. (41) support this theory, Einstein worked (42) what is probably the most important equation in the history of mathematics. It was (43) no means a complicated equation, but in fact a very simple (44) . Here it is: E=MC2. Turned into simple language, Einstein’s equation means that energy(E) is equal (45) mass(M) multiplied by the speed of light(C) squared(C2).
串是由零个或多个______组成的。
层次数据库模型的结构是一棵______，树的结点是记录类型。
2009年5月，某零售企业的部分陶瓷供应商陆续收到来自其零售企业的审核函。函中表示零售企业将派出专员到陶瓷厂家进行现场审核，审核目的主要是评估工厂是否符合相应的要求。据悉，审核结果很有可能将决定该陶瓷厂家是否能与零售企业续约。 审核结束后，应对供应商的( )以及相关使用经验、服务和技术支持能力情况进行综合评估后，才能确定是否选择。
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