C

For thousands of years, we have looked for ways to measure time. Early humans found that the regular movements of the sun, the earth, the moon and the stars made good ways to measure time. The rising and setting of the sun were used to distinguish day from night.

But, eventually, people needed to tell time more accurately, or exactly. So, by using the sun's position in the sky, they divided the day into dawn, morning, midday and evening.

Then it was noted that the sun cast a changing shadow as it moved across the sky. Time could be told more accurately by setting up a stick and making the positions of the sun's shadow. It was the ancient Greeks(古希腊人)who divided each position of this "sundial(日)" into hours. But the sun doesn't always shine. So far the past 6,000 years, many other ways of keeping time have been tried. Slow-burning candles were divided into hours, and the hourglass was invented. When all the sand in the top of an hourglass has shifted(过滤)， to the bottom, an hour has passed.

Later, the pendulum(钟摆), with its regular back-and-forth 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 battery-operated quartz clocks, digital clocks and clocks run by electrical turning forks(音叉)and tiny atoms(原子). These atomic clocks are the most accurate clocks ever invented. The exact time can be kept to within one second a century.

49.When something is very exact, it is said to be __________.

A.slow B.accurate C.fast D.of poor quality

50.Humans in the old days used ________to tell day from night.

A.the rising and setting of the sun B.battery-operated quartz clocks

C.atomic clocks D.digital clocks

51.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 candle in the sunlight

D.watching the stars

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

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