题目

Insomnia is the inability to fall asleep or sleep restfully. It is a problem to nearly everyone at some time. A person may be awake for an hour in the middle of the night and then, next morning, feel that he hardly sleep all night. Normally, worry about not getting enough sleep is the worst aspect of most insomnia. If insomnia happens again and again, however, it is important to find its causes and try to correct them, with the help of a doctor if necessary.        Any of a large number of simple factors may make it difficult for someone to fall asleep or remain asleep. His mattress （下列不具有细胞结构的是（ ）A．① B．② C．③ D．④