单项选择题

Rivers have been important to man ______.

A．since the last century
B．for a very long time
C．since a few years ago
D．since a few hundred years ago
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单项选择题
According to Flextime system workers need not ______.
A．work by the same timetable
B．work all the weekdays
C．go to work at weekends
D．go to work in the early morning
单项选择题
Key time is a period when ______.
A．visitors come to the plants
B．all workers must be at work
C．employers go round in the workshops
D．rush hours are over
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