单项选择题

In July 1994, the comet(彗星) Shoemaker-Levy 9 smashed into the planet Jupiter. For the first time, humans were able to witness exactly what happens when a celestial(天空的) body collides with a planet and it quickly became clear that survival was no longer entirely a question of being the "fittest". A new factor had been introduced into evolution: the ability to survive a collision between the earth and an asteroid (小行星) or comet.
To most people, the risk remains academic. With all the dangers humans face -- sickness, accidents- it is understandable that people don’t take seriously the risks posed by something that hasn’t happened for 65 million years and may not happen for another 65 million years.
However, many scientists believe that collisions between the earth and celestial bodies cannot be regarded as "just another risk". The main reason for this is that no other disaster -- except perhaps a nuclear war -- has the potential to destroy human civilization completely. Even the worst floods and earthquakes affect only a very small percentage of the earth’s surface and population. But the effects of an impact caused by a celestial body of just ten kilometers in diameter would make humans extinct, along with most of the world’s other animals and plants.
The danger comes from asteroids and comets which cross the earth’s orbit. Asteroids pose a greater danger because they are more numerous. Those less than 100 meters in diameter are not usually regarded as a threat bemuse most are destroyed by heat as they enter the earth’s atmosphere and so never reach the ground. It is those asteroids with diameters of one kilometer or more which pose the greatest threat.

According to the passage, asteroids which can do real harm to the earth are （）.

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