Although I didn't have any interest in being a school bus driver, I became one. Many years later, I was still working on the school bus. There are so many good reasons. Firstly, the talk brought me different kinds of information. I also got to hear all the family secrets of the newcomers. I have a toy bear who was a great comforter(安慰者) for the little ones. The kids got to look after him on the trip to school and he waited patiently for them to get on for the trip home.
      Our bus would often pass a place where we could see some alpacas (羊驼) in front of a farm house. Little Michael would cry excitedly from the seat, “Look, Miss! Look at the camels!” He used to put his head out of the bus window. When I warned him that a little insect would come into his nose if he wasn’t careful, he smartly gave up the habit.
      I remember a little girl got on the bus one morning 5 years ago and sobbed(抽泣) all the way to school. We arrived at the school gate. As she waited to get down the bus, she threw her arms around my neck and cried that her grandfather had died the night before.
    A school bus driver needs to keep one eye on the road and the other on the mirror to watch what the little ones are doing down the back.
Now, it's important to think how many lives I've been responsible(负责的) for, and how many kilometers I've driven in order to get my passengers home safely during the 20 years. I've enjoyed it, but it’s time to rest and park my school bus.

如图为空气成分示意图（按体积计算），其中“a”代表的是（  ）

如图为空气成分示意图（按体积计算），其中“a”代表的是（  ）