问题 选择题

两条异面直线在同一平面的正投影不可能是(  )

A.两条平行直线

B.两条相交直线

C.一个点和一条直线

D.两个点

答案

答案:D

分析:根据异面直线的定义,分别分析投影面与已知的两条异面直线及异面直线的公垂线之间的关系,排除可能出现的情况,可得答案.

解答:解:如果投影面与两条异面直线的公垂线平行,且两条异面直线与投影面均不垂直,此时两条异面直线的投影为两条平行线

如果投影面与两条异面直线的公垂线垂直,此时两条异面直线的投影为两条相交直线,

如果投影面与两条异面直线的公垂线平行,且有一条直线与投影面垂直,此时两条异面直线的投影为一点和一条直线

但两条异面直线在同一平面的射影不可能是两个点

故选D

点评:本题考查的知识点是空间中直线与直线之间的位置关系,熟练掌握空间异面直线的几何特征是解答的关键.

阅读理解

阅读理解。

     Cities alarmed by deaths and injuries of pedestrians are taking efforts to make crosswalks safer for people

on foot, especially seniors and children who need more time to cross streets.

     A pedestrian is killed in a traffic accident in the USA every 110 minutes; one is injured every nine minutes,

according to official data. Crosswalks can be especially dangerous for the elderly. Among people 70 and older,

36% of pedestrian deaths in 2006 occurred in crosswalks, compared with 21% of those younger than 70,

according to the Insurance Institute for Highway Safety.

     The Federal Highway Administration (FHWA) advise that next year states increase by nearly 15% the

amount of time traffic lights provide for pedestrians to cross the street after the flashing orange hand appears.

FHWA spokesman Doug Hecox says reasons for the change include an aging population that needs more time

to cross, health-conscious Americans walking more, children encouraged to walk to prevent getting overweight

and high gas prices pushing people to walk instead of drive.

     Pedestrian deaths went down by 12% from 5,449 in 1996 to 4,784 in 2006, Bur among those in 2006, 471

were killed in crosswalks, down slightly from 488 ten years earlier, the National Highway Traffic Safety

Administration (NHTSA) says.

1. Which of the following is true according to the text?

A. Among 100 pedestrian deaths there were 21 people younger than 70.

B. Old people are more likely to meet with accidents happened per hour.

C. Traffic accidents killed more old people than young people.

D. About seven traffic accidents happened per hour.

2. What is FHWA's suggestion to states?

A. Fixing more traffic lights

B. Providing more crosswalks

C. Giving pedestrians more time to cross streets

D. Drivers don't give way

3.  What’s the cause of the crosswalk safety problem according to the text?

A. There’re many cars and buses on the road.                         

B. Pedestrians are careless.

C. Crosswalks are more crowded than before.                        

D. Drivers don’t give way.

4. The report from NHTSA suggests that _____.

A. fewer people were injured in crosswalks

B. crosswalk safety has been greatly improved

C. much has been done to reduce traffic accidents

D. pedestrian deaths in crosswalks remain a serious problem

问答题 简答题