某有机物R在足量氧气中完全燃烧后的化学方程式为：2R+13O2
8CO2+10H2O．下列关于该反应的说法正确的是（ ） 点燃 .
A．该反应中R的化学式为C4H10
B．该反应后产生的CO2可用生石灰干燥
C．将反应后生成的CO2溶于水，所得溶液的PH＞7
D．该反应为置换反应
A、根据质量守恒定律，反应前后原子种类和个数都不变，由方程式知生成物中含有的原子种类及原子个数为C，8；H，20；O，26；已知反应物中含有的原子种类及原子个数为O，26；比较分析可知R中含有C和H元素，其原子个数分别为4和10，故R的化学式为C4H10，故A说法正确；
B、反应后产生的CO2中有水，水会和生石灰反应生成氢氧化钙，二氧化碳能和氢氧化钙反应，因此不可用生石灰干燥，故B说法错误；
C、反应后生成的CO2溶于水，二氧化碳和水反应生成碳酸，碳酸溶液显酸性，因此所得溶液的PH＜7，故C说法错误；
D、该反应生成物是两种化合物，不属于置换反应，故D说法错误．
故选A．
注意：每空不超过3个单词。 Frederick M. Hess is the director of education policy studies in American. He says that a long summer vacation doesn't make sense in today's world. In a summer vacation, millions of kids spend valuable time sending messages, watching TV, playing video games and doing shopping in the mall. These activities are putting their futures at risk. Summer vacation once made sense in the past when you didn't need an education to get a good job. But now things have changed. For today's students, academic(学术的) skills are important to students' future success, but such skills are affected in the summertime. Most American schools offer up to thirteen weeks summer vacation. Summer vacation also causes challenges for today's families. In the 1960s, more than 60% of families had a stay-at-home Mom. Now two-thirds of American children live in households where every adult works. For these families, summer vacation can be more burden than break. Someone must watch the kids. “A longer school year does not have to be hard and boring work. Instead, it should allow teachers to give richer and more imaginative lessons,” Frederick M. Hess suggested, “Schools would have more time to devote to athletics, languages, music and the arts”. Summer vacation can be a grand thing. But in the 21st century, it may also be outdated. Title: Long Summer Vacation
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