Charlie studied in a famous college for four years. He studied hard and did well in all his subjects. He hoped to become a good teacher. This year he left the college and began to work in a middle school. He likes his students and is strict with them. He does his best to make his classes lively and interesting.

    One day he carefully explained a chemical reaction to the students of Grade 2 in the chemistry lab.

    “Be careful, everyone,” he said loudly. “Before I make the experiment, there're twenty five atoms of carbon (碳原子), but after I finish it, there're twenty four atoms of carbon left!”

He stopped to watch the classroom and hoped his students would go on explaining it. But the young men looked at each other and nobody answered him. He had to ask,

    “What happens? What makes the atom lost?”

    The classroom was very quiet and none of the students looked at their teacher.

    “Who can tell us where it has gone?”

    Suddenly a soft voice came from the back row, “We did not see anybody leave the lab!”

在北方的农村，往往有贮藏蔬菜瓜果的地窖，当人们进入地窖之前往往要先用烛火做实验，如果烛火熄灭，人们就不会进入地窖。烛火在地窖中熄灭，说明地窖中缺少____，此时，地窖中的蔬菜瓜果的呼吸作用比较____（填“强或弱”）。