sun's shadow. It was the ancient Greeks who divided each position of this "sundial(日晷)" into hours.

But the sun doesn't always shine. So，for the past 6,000 years，many other ways of keeping time have been tried. Slow­burning candles were divided into hours，and the hourglass was invented. When all the sand in the top of an hourglass has shifted to the bottom，an hour has passed.

Later，the pendulum(摆钟)，with its regular back­and­forth movement of weights，was used to move the hands on a clock. Pendulums are still used in grandfather clocks.

Today，even more accurate clocks are in use，such as battery­operated quartz clocks（石英钟），digital clocks，and clocks run by electrical tuning forks and tiny atoms. These atomic clocks are the most accurate clocks ever invented. The exact time can be kept to within 1 second a century.

21.Humans in the old days used ________ to tell day from night.

A．the rising and setting of the sun

B．battery­operated quartz clocks

C．atomic clocks

D．digital clocks

22.A sundial works by ________.

A．tracking the movement of the stars around the sun

B．marking a shadow cast by the moving sun

C．burning candles in the sunlight

D．watching the stars

23.Which of the following was NOT used in the past to measure time?

A．An hourglass. B．A sundial. C．Digital clocks. D．A pendulum.

24.Which of the following ways of measuring time is ordered from the least accurate to the most accurate?

A．Sundial，pendulum，sun.

B．Atomic clock，quartz clock，sundial.

C．Sun，pendulum，atomic clock.