水是国家战略性资源，下列物质对水质不会产生严重污染的是    （   ）

词在填入前需进行词形转换，每个单词只能用一次，每空只能写一词。

    Norman Bethune is one of the most famous ____ for Chinese people. He was not Chinese, but he gave his life to helping Chinese people.

    Norman Bethune was born in 1890, in Canada. He ____ a doctor in 1916. In 1938 he came to China. There were few doctors then, so he had to work very ____. Once, he worked for 69 hours without stopping, and saved 112 people. Although he cut his hand, he did not stop to ____ care of it. In the end, he died from the cut.

    Dr Bethune's work with the Chinese soldiers made ____ a hero in China. He will be remembered by Chinese people for ever.

一千五百年前的《孙子算经》中有“鸡兔同笼”问题：“今有鸡兔同笼，上有三十五头，下有九十四足，问鸡兔各几何？设鸡有x只，根据题意可得方程为（　　）