19、One day when Isaac Newton was reading a book under an apple tree on the farm，an apple fell down and hit him on the head．For most people that would be the end of the story，but not for Newton．Why did the apple fall out of the tree? Does everything fall? What makes things fall? Can anything stop things from falling? Are the sun，moon，and stars falling? Why don't they ever hit the ground?

Newton spent many years answering these questions by thinking and doing experiments．He made up the law of gravity（引力定律）．According to this law everything pulls everything else to itself by a force（力）called gravity．How strong that force is depends on how heavy the things are and how close together．

Newton's law of gravity not only explained how things fall on earth，but how planets move around the sun and how moons move around planets．A friend of his，Edmund Halley，decided to try Newton's theory（理论）on comets（彗星）．People had been studying comets for hundreds of years but failed to understand them，so he decided to study their reports and compare them to Newton's theory．

Up till then people had thought that comets just came and went，and that nobody could know when or why．But Newton's law of gravity gave rules that Halley could use to study the records of comets．He found some reports of a big bright comet that he was sure was the same one，coming back every 75 years．He predicted（预测）when it would come back next．

Halley's prediction came true．Tile comet appeared at the right time as he predicted using Newton's ideas．This was enough to convince anybody who still didn't believe Newton．Halley's Comet has come a few times since then，always right on time．You'll be able to see it on its next trip near the sun and earth when you're old enough to be a grandparent．

1．The passage starts with a story to show _______．

A．it was such a funny one

B．Newton liked to eat apples

C．how much Newton liked reading

D．what led to Newton's discovery of gravity

2．How strong gravity is depends on _______ of the things according the Newton’s theory．

A．color and distance（距离）                      B．weight and distance

C．size and weight                                          D．size and color

3．According to the writer，Newton was especially different from others in that _______．

A．he liked reading under apple trees              B．he liked to find out how things worked

C．he was very quiet and patient                             D．he was so much interested in science

4．The underlined word convince in the last paragraph probably means “make people _______”．

A．believe                 B．doubt                   C．understand             D．relax

5．Which of the following is true about Edmund Halley?

A．He discovered a big bright comet

B．He helped Newton to create his theory．

C．He predicted when the same comet would come back．

D．He studied the comet for 75 years