17.In old times,man considered lightning to be one of the greatest mysteries of nature.In fact,a single flash of lightning 1.6 km long has enough electricity to light one million lamps.
The American scientist,Benjamin Franklin,was the first to suggest that lightning is in fact electricity formed between the Earth and clouds high above.It happened in 1752.In the same year,he also built the first lightning rod to protect buildings from being struck by lightnin g.
Modern science has discovered that lightning is very long and powerful.A lightning between a cloud and the Earth may be as long as 13 km,and travels at a speed of 30 million meters per second.Scientists have the opinion that there are about 2 billion flashes of lightning per year.In the United States alone,it kills almost one person per day.
The safest place to be in during an electrical storm is a closed car.Outside,one should go close to low ground for cover and not stop under a tree.Also one should stay out of water and away from metal fences.Inside a house,people should keep away from open doorways and windows and not touch wires or metal things.
29.In the USA,D.
A.every person can see lightning every day
B.at least one person is killed by lightning every day
C.there are about 6 million flashes of lightning per year
D.about one person per day dies from lightning
30.During a lightning storm,it would be best if youC.
A.stand under a tree
B.go outside
C.stay in a closed car
D.stop near windows
31. A lightning rod canA.
A.prevent accidents caused by lightning
B.stop lightning
C.protect building from being washed away
D.delay the coming of lightning.