在2H2+O2
2H2O 反应中，反应前后发生改变的是（ ） 点燃 .
A．原子种类
B．元素种类
C．分子总数
D．原子总数
由反应的化学方程式2H2+O2
2H2O，可知该反应中氢分子、氧分子、水分子分子个数比为2：1：2，每个氢分子由2个氢原子构成、每个氧分子由2个氧原子构成、每个水分子由2个氢原子和1个氧原子构成，则可判断：反应前后原子种类、原子数目及元素种类均不变，而分子种类由反应前的氢分子和氧分子变化为反应后的水分子，分子的数目由3个变为2个． 点燃 .
故选C．
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| Have you ever imagined that you could be cloned (克隆) like Dolly,the sheep,in the not too distant future? The news that a human embryo (胚胎) has been 1 cloned for the first time has caused mixed reactions(反应). The 2 was carried out by scientists from the Advanced Cell Technology Inc(ACT),in Massachusetts,US. The group 3 the news on November 25. This is not a cloned human being. "A 4 is alive,it walks,it breathes,"said Jamie Grifo,an expert on the study of cloning at New York University School of Medicine. "This is a set of cells in a lab that will be used to 5 someone's life." Such research could lead to treatment for 6 such as heart disease,AIDS and even cancer, 7 scientists. Despite high hopes from other scientists,the news 8 concerns immediately from religious and political leaders. Several 9 in the US do not allow human cloning. President George W.Bush also made it clear that he is 10 any type of human cloning. However, the 11 at ACT said they have no interest at present in developing an early embryo into 12 . Animals have been cloned repeatedly since Dolly,the sheep, 13 in 1997.And there were no real technical 14 to stand in the way of scientists making a cloned human embryo. This time the research group used traditional cloning technology with a human 15 . 16 it was given DNA from an adult cell,the egg began to 17 . 18 it was stopped from becoming a baby-at a stage in which it was 19 a ball of cells. The 20 technology has been used to clone sheep,cattle and monkeys. | ||||
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