已知反比例函数的图象过点M（－1，2），则此反比例函数的表达式为
实验室配制50g溶质质量分数为6％的硝酸钾溶液。现准备如下实验用品，请回答下列问题：
选择合适的短语填空。
on air, in the background, show sb．around, thank you for, the end of
光在____物质里传播的路线是直的；光在真空中的传播速度最大，数值是____米/秒．“立竿见影”的现象表明光在空气中是沿____传播的．
You are late again. What’s your e____ （ 理由） for being late this time ？Did you know who d____ (发现) America?It may be s____ (晴朗的) tomorrow.Most people can greatly improve the q____ (质量) of life.Many tall buildings have a____ (已经) been built in our city.
