The letter said that there was an______, but I couldn' t find where it was.

A．Because few people like to reverse the cycle of sleep aud wakefulness.
B．Because sleep normally coincides with the hours of darkness.
C．Because people are required to work at night in some fields of industry.
D．Because shift work in industry requires people to change their sleeping habits.

A．the inconveniences brought about to the workers by the introduction of automation
B．the disturbance of the daily life cycle of workers who have to change shifts too frequently
C．the fact that people working at night are often less effective
D．the fact that it is difficult to find a number of good night workers