正常的雨水因溶有____而呈弱酸性，通常将pH小于____的雨.雪.雾.雹.露.霜等称为酸雨. 、NOx、含氯化合物等物质的排放是形成酸雨的主要原因.我国主要是____型酸雨，其形成过程有两种途径：一种是 在空气中的飘尘做催化剂的作用下被氧气氧化为 ，溶于水生成硫酸，写出有关的反应化学方程式____、____；另一种是 溶于水生成亚硫酸，再被空气中的氧气氧化成硫酸，写出有关的反应化学方程式____、____.由于 的氧化非常缓慢，所以后一种途径是其主要形成过程.

阅读下列短文, 从下面每小题的A、B、C、D四个选项中选出最佳选项。

There are some great women scientists in the world. The world would certainly not be so wonderful without their hard work.

Here is a list of some top women scientists and their achievements.

◆Marie Curie

      Curie was born in Poland on November 7, 1867. She was a pioneer in the field of radioactivity(放射). She was also the first person to win two Nobel Prizes, one in physics and the other in chemistry.

◆Rosalind Elsie Franklin

     Franklin was born on July 25, 1920 in London. She graduated from Cambridge University. She played a great role (角色) in the discovery of the structure(结构)of DNA.

◆Annie Jump Cannon

      Cannon was born on December 11, 1863. She was an American astronomer. She divided more than 500,000 stars into different kinds and discovered more than 300 stars in her life.

◆Sally Kristen Ride    

      Ride was born on May 26, 1951. She was an American astronaut. She was the first American woman to travel into outer space.

◆Grace Murray Hopper

      Hopper was born on December 9, 1906 in New York. She was one of the earliest computer programmers and a leader in the development of computer software.

一个不透明的袋子中装有2个白球和若干个黑球，它们除颜色外完全相同，从袋子中随机摸出一球，记下颜色并放回，重复该实验多次，发现摸到白球的频率稳定在0.4，则可判断袋子中黑球的个数为（　　）

2015年6月1日，“东方之星”客船在湖北监利县遭遇强台风翻沉．潜水员潜入12m深水里搜救落水乘客，此时潜水员受到水的压强是（已知ρ_水=1.0×10³kg/m³）（　　）

一张正方形纸对折后再对折，写出一个田字，打开后看见（  ）个田字。。