A) It is very unlikely but the danger exists.
B) Such a collision might occur once every 25 years.
C) Collisions of smaller asteroids with Earth occur more often than expected.
D) It‘s still too early to say whether such a collision might occur.
23.What do people think of the suggestion of using nuclear weapons to alter the courses of asteroids?
A) It sounds practical but it may not solve the problem.
B) It may create more problems than it might solve.
C) It is a waste of money because a collision of asteroids with Earth is very unlikely.
D) Further research should be done before it is proved applicable.
24.We can conclude from the passage that ________.
A) while pushing asteroids off course nuclear weapons would destroy the world
B) asteroids racing across the night sky are likely to hit Earth in the near future
C) the worry about asteroids can be left to future generations since it is unlikely to happen in our lifetime
D) workable solutions still have to be found to prevent a collision of asteroids with Earth
25.Which of the following best describes the author‘s tone in this passage?
A) Optimistic.
B) Critical.
C) Objective.
D) Arbitrary.
答案：21.B 22.A 23.B 24.D 25.C