In the second paragraph, the third means______.

A．Machines can help solve the problem of working at night.
B．A person can work more efficiently by frequent changes between day shifts and night shifts.
C．C ) The adaptation to night work can be destroyed if the persons return to normal habits.
D．Permanent night workers are more likely to have such symptoms as digestive disorder.

A．keep more in the store
B．give more rights
C．keep more interested
D．maintain more chances