Sixty-five million years ago, an asteroid   that crashed on Earth led to the disappearance of the largest animals that have ever walked our planet - the dinosaurs. At least, this is what some scientists believe. But that accident happened so long ago. People have come to believe that we are free of threats from asteroids and everything else from space. However, what happened on Feb 15 , 2013 was a reminder that we're just as _ as the dinosaurs once were. Two objects from space - a meteor   and an asteroid called 2012 DA14 -  visited Earth's atmosphere on the same day. The former fell in Russia, injuring 1,200 people, while the latter passed by Earth at a record-setting close distance. Scientists had been expecting the asteroid since last year, but the meteor was a surprise. NASA's telescope system only detects asteroids at least 50 meters in diameter  , which is just about the size of 2012 DA14. But the meteor was much smaller, which made it harder to spot. But what if we do spot an asteroid that is headed right for Earth? There are several possible ways in which dangerous asteroids could be made to change its orbit. Which method is best depends on several factors and most importantly  -  how much time we have to stop it. If there is enough time before the hit, we can send off a heavy spacecraft to travel alongside the asteroid. The gravity from the spacecraft would gradually change the rock's orbit. Besides that, scientists could one day use sun-powered lasers to either make asteroids disappear or change their course. If there's not enough time, we'll have to go after the asteroid with a spacecraft and change its orbit with a crash. Finally, if things are truly desperate, there will be only one choice left -  to use a nuclear bomb. That could turn the asteroid into a meteor shower, which would be even more dangerous. What is the main idea of the article?
Learning about asteroids and meteors. The threats of objects from space and possible solutions. NASA's latest technology to discover visitors to the Earth from space. How to measure the damage of collisions from asteroids and meteors.
Question: Sixty-five million years ago, an asteroid   that crashed on Earth led to the disappearance of the largest animals that have ever walked our planet - the dinosaurs. At least, this is what some scientists believe. But that accident happened so long ago. People have come to believe that we are free of threats from asteroids and everything else from space. However, what happened on Feb 15 , 2013 was a reminder that we're just as _ as the dinosaurs once were. Two objects from space - a meteor   and an asteroid called 2012 DA14 -  visited Earth's atmosphere on the same day. The former fell in Russia, injuring 1,200 people, while the latter passed by Earth at a record-setting close distance. Scientists had been expecting the asteroid since last year, but the meteor was a surprise. NASA's telescope system only detects asteroids at least 50 meters in diameter  , which is just about the size of 2012 DA14. But the meteor was much smaller, which made it harder to spot. But what if we do spot an asteroid that is headed right for Earth? There are several possible ways in which dangerous asteroids could be made to change its orbit. Which method is best depends on several factors and most importantly  -  how much time we have to stop it. If there is enough time before the hit, we can send off a heavy spacecraft to travel alongside the asteroid. The gravity from the spacecraft would gradually change the rock's orbit. Besides that, scientists could one day use sun-powered lasers to either make asteroids disappear or change their course. If there's not enough time, we'll have to go after the asteroid with a spacecraft and change its orbit with a crash. Finally, if things are truly desperate, there will be only one choice left -  to use a nuclear bomb. That could turn the asteroid into a meteor shower, which would be even more dangerous. What is the main idea of the article?
A. Learning about asteroids and meteors.
B. The threats of objects from space and possible solutions.
C. NASA's latest technology to discover visitors to the Earth from space.
D. How to measure the damage of collisions from asteroids and meteors.
Answer: