问题 填空题

如图所示的使用测电笔的方法正确的是图______。(选填“甲”或“乙”)

答案

题目分析:用测电笔来辨别火线与零线.使用测电笔时,笔尖接触要检测的导线,手接触笔尾金属体,才能在接触火线时氖管发光,接触零线时氖管不发光。使用测电笔时,笔尖接触被检测的导线,如果手不接触笔尾金属体时,笔尖无论接触火线还是零线,氖管都不发光.所以手一定要接触笔尾的金属体,氖管发光时接触的是火线,氖管不发光时接触的是零线。

点评:掌握测电笔的正确使用,手一定接触笔尾金属体,手一定不能接触笔尖金属体。

阅读理解

                              D

The year is 2094. It has been announced that a comet (彗星) is heading towards the Earth . Most of it will miss our planet , but two pieces will probably hit the southern half of the Earth .

On 17 July , a piece four kilometers wide enters the Earth’s atmosphere (大气层) with a massive explosion . About half of the piece is destroyed , but the remaining part hits the South Atlantic at 200 times the speed of sound . The sea boils and a huge hole is made in the sea bed . Huge waves are created and spread outwards from the hole . The wall of water , a kilometer high , rushes towards southern Africa at 800 kilometers an hour . Cities on the African coast are totally destroyed and millions of people re drowned .

Before the waves reach South America , the second piece of the comet lands in Argentina . Earthquakes and volcanoes are set off in the Andes Mountains . The shock waves move north into California and all around the Pacific Ocean . The cities of Los Angeles , San Francisco and Tokyo are completely destroyed by earthquakes . Millions of people in the southern half of the earth are already dead, but the north won’t escape for long . Because of the explosions , the sun is hidden by clouds of dust , temperatures around the world fall to almost zero .Crops are ruined . The sun won’t be seen again for many years . Wars break out as countries fight for food . A year later , no more than 10 million people remain alive .

Could it really happen ? In fact , it has already happened more than once in the history of the Earth . The dinosaurs were on the Earth for over 160 million years . Then 65 million years ago they suddenly disappeared . Many scientists believe that the Earth was hit by a piece of object in space . The dinosaurs couldn’t live through the cold climate that followed and they died out . Will we meet the same end ?

小题1:What is mainly described in the passage ?

A.A historic discovery .

B.An event of imagination

C.A research on space .

D.A scientific adventure .小题2:When the first piece hits the South Atlantic , it causes           .

A.an earthquake

B.damages to cities

C.an Earth explosion

D.huge waves小题3:Why can’t the northern half of the earth escape for long ?

A.Because the land is covered with water

B.Because the light and heat from the sun can not reach the earth .

C.Because people there can not live at the temperature of zero .

D.Because wars break out among countries .

单项选择题