题目

There was enough money to pay for schools and food, but not enough for a room and food. I had to look for a part-time(兼职的) job. Then a man called Jack, asked me to look after his grandparents.My first discovery(发现) was his grandmother's great love of music. She spent hours playing the piano. Some days later she began to teach me the art.Grandma was moved by my ability(能力) and encouraged(鼓励) me to continue(继续). On weekends, their house was full of happiness. They played and sang happily.One day after a few months, as I was running back to his house, the phone rang. “Tom,澄清石灰水长期敞口放在空气中会变浑浊，这是因为空气中含有（ ）A．氧气B．二氧化碳C．氮气D．一氧化碳