A special laboratory at the University of Chicago is busy only at night. It is a dream laboratory where researchers are at work studying dreamers. Their findings have proved that everyone dreams from three to seven times a night, although in normal life a person may remember none or only one of his dreams. While the subjects--usually students--sleep, special machines record their brain waves and eye movements as well as the body movements that signal (信号显示) the end of a dream. Surprisingly, all subjects sleep soundly. Observers report that a person usually fidgets (烦躁不安 ) before a dream. Once the dream has started, his body relaxes and his eyes become more active, as if the curtain had gone up on a show. As soon as the machine shows that the dream is over, a buzzer (电铃声) wakens the sleeper. He sits up, records his dream, and goes back to sleep perhaps to dream some more. Researchers have found that ii the dreamer is awoken immediately after his dream, he can usually recall (回忆) the entire dream. If he is allowed to sleep even five more minutes his memory of the dream will have disappeared. The researchers were surprised to find that______. A: people sleep soundly while they dream B: dream memories are often incomplete C: sleepers fidget while dreaming D: dreamers can record their own dreams