若方程组$\left\{\begin{array}{c}ax+y=5\\ x+by=-1\end{array}\right.$的解为$\left\{\begin{array}{c}x=2\\ y=1\end{array}\right.$，则点P（a，b）在第 ____象限．

利用家庭生活用品可以对科学知识进行学习和探究。食盐、食醋、纯碱均为家庭厨房中常用的物质，利用这些物质不能完成的实验是(    )

世界资本主义经济危机中有一种奇特的现象：美国有1040万英亩的棉花被铲倒在地；倒入密西西比河的牛奶．使这条河成了“银河”；巴西有2200万袋咖啡豆被烧毁或倒入大海；丹麦毁灭了11.7千头牲畜。这是(     )

走进“化学村”.“化学村”的布局如下：

   You may never be in an emergency situation. But if it happens, you should know how to get help. The telephone book in the United States has emergency numbers on the inside front cover. Look at the following table from the Boston telephone book. Notice that the numbers for the police and the fire department (部门) are the same. And it is an easy number to remember. If you are too unhappy or excited to remember any numbers at all, you can simply dial (拨打) “0” for an operator (接线员) in any emergency.