Directions: Read the following text and

Directions: Read the following text and choose the most suitable heading from A – F for each paragraph. There is one extra heading which you do not need.

A．Human efforts to prevent disastrous collisions B．Recent alarms of asteroid(小行星) strikes． C．Asteroids’ movement around the sun． D．Asteroid strikes against Earth． E．Past disastrous asteroid strikes． F．The cause of asteroid strikes．

76._________________
Astronomers estimate that there are about l,100 asteroids--or space rocks--with the potential to cause a planet-wide disaster in a collision with Earth．And since large asteroids have collided with Earth many times in the past，scientists say it is certain that Earth will be hit again．It is only a question of when． 
77. _________________
The asteroid that ended the age of the dinosaurs hit the Earth 65 million years ago．It was at least six miles(10 kilometers)wide．But smaller asteroids can also devastate （毁坏）the earth．Scientists estimate that a collision with an asteroid even one—tenth this size would kill at least 25 per cent of Earth’s population．Less than a hundred years ago，for example，an asteroid only 330 feet(100 meters)wide exploded in Siberia．It completely destroyed half a million acres of forest．
78．_________________
Asteroids come from a belt of tens of thousands of space rocks in orbit around the sun．They normally travel between Mars and Jupiter，but some smaller rocks are affected by the gravitational pull of Mars，Jupiter or Saturn and their orbits are stretched．Sometimes this change puts them on a path that crosses Earth’s orbit．This sets up the possibility of collision with Earth．
79．_________________
In l986，a potentially dangerous asteroid missed Earth by only six hours．Scientists only found out about the danger after it had passed．Many more asteroids have collided with Earth over the years．We can still see their marks today — large craters in the ground．
80．_________________
NOW there is a new--found seriousness about asteroid strikes．Astronomers are engaged in a painstaking search for all asteroids that threaten Earth．And the United States has a project called NEAR that involves orbiting and studying the asteroid Eros，the second largest asteroid near Earth．Studying the composition of the asteroid provides important information hat may allow an asteroid to be exploded or knocked of course to prevent it from colliding with Earth．