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 ,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!" The students couldn't answer Charlie's questions because   _   .
none of them had seen the atoms they didn't understand their teacher the chemical reaction was too complicated they all wanted to drop the subject