A sand clock is an even better clock. If you had fine dry sand in a glass shaped like a funnel（漏斗）, you would have what is called an hour glass. The sand in the hourglass goes from the top part to the bottom part in exactly an hour. When the hourglass is turned over, the sand will take another hour to go back again.

23. The Eskimos in the far north can't use the sun for a clock because ______.

A. the sun there never goes down in winter

B. There are long dark winters and long light summers in the far north

C. they know very little about the sun

D. It's too cold for them to go out to watch the sun

24. The underlined word "tides" in the paragraph 3 means ______.

A. 海啸 B. 洋流 C. 波浪 D. 潮汐

25. In which part of a newspaper can you most probably read this passage?

A. business B. news C. advertisement（广告） D. science

26. What's the best title for the passage?

A. Useful Machine to Tell Time

B. Different Ways to Tell Time

C. The Development of the Clock

D. The History of the Clock

　　　　　　　　　　　　　　　　　　　　　D

Scientist Florence Wambugu works with farmers in Kenya, a country in East Africa. She helps them grow bigger and better crops. Wambugu is especially interested in finding simple ways to produce more food.

In the past ten years, Wambugu has spent much of her time studying sweet potatoes, which are an important food in her part of Kenya. A virus（病毒）kept attacking the plants. It stopped the sweet potatoes from growing well. Because of the virus, some farmers lost three quarters of their crops.

Wambugu went to war against the virus. Her search for a way to save the sweet potatoes led to a lab in St. Louis, Missouri.

The lab mainly works on genes（基因）, the chemical "computer programs" found in