选词填空。

       hers     his    theirs

某厂一月份的总产量为500吨，三月份的总产量达到为720吨。若平均每月增长率是，则可以列方程（   ）

      It was the first day of biology class. The teacher wanted to give a good1  to his class. He wanted to  2  his students’ eyes to the world of biology.
      He knew that 3  young students was not easy, so he worked 4  and tried to find a good way to teach this class.5  thinking for a long time, he made a 6  at last. He decided to show the kids something7 and teach them a lesson at the same time.
      He asked his students to 8  carefully. Then he began. He put a worm(虫子) into a glass of water. The worm 9 around in the water. Then he put a second worm into a glass full of alcohol(酒精). It died at once. He asked the students if anyone knew about 10 they learned from it. A boy said, “You showed us that if we drink alcohol we won't have worms.”

影响声音传播速度的是（　　）