市售珍珠王牙膏的摩擦剂是纯度极高的轻质碳酸钙粉末，它是以石灰石为原料经过下列转化制得的：石灰石→生石灰→X→碳酸钙，根据上述转化关系式推断：X的主要成分化学式为（　　）

实验室模拟合成氨和氨催化氧化的流程如下：

已知实验室可用饱和亚硝酸钠(NaNO₂)溶液与饱和氯化铵溶液经加热后反应制取氮气。

设某批电子手表正品率为$\frac{3}{4}$，次品率为$\frac{1}{4}$，现对该批电子手表进行测试，设第X次首次测到正品，则P(X=3)等于(    )

明史专家吴晗在《朱元璋》一书中写道：“在宋以前有三公坐而论道的说法……到宋朝便不然了。从太祖以后，大臣上朝在皇帝面前无坐处，一坐群站，……在明代，不但不许坐，站着都不行，得跪着说话了。”君臣关系从坐而站而跪这一现象说明（   ）

阅读短文，下列问题

 B

American boys and girls love to watch television. Some children spend six hours a day in school and four to six hours a day in front of the television set. Some even watch television for eight hours or more on Saturday.

     Televisions are like books or films. A child can learn bad things and good things from them. Some programmes (节目) help children to understand (理解)the news, others show people and places from other countries or other time in history. With television a child does not have to go to the zoo to see animals to the ocean to see a ship. Boys and girls can see a play, a concert or a game at home.

     Television brings many places and events into our homes. Some programmes show crime (犯罪) and other things that are bad for children, so parents sometimes help them to find other interesting things to do.