Humans: It's time to give up the earth or face extinction . It could be the plot of a summer blockbuster , but this prediction isn't science fiction. It's a glimpse  of the future, according to famous British scientist Stephen Hawking. "I see great dangers for the human race," he said in an interview with global forum  Big Think. "There have been a number of times in the past when its survival has been a question of touch-and-go." Hawking, an award-winning physicist, says humans must evacuate our home planet within the next two centuries and expand into space to survive. "I believe that the long-term future of the human race must be in space," he said. "It will be difficult enough to avoid disasters on our planet in the next hundred years, let alone the next thousand, or million." Hawking, who was awarded the Presidential Medal of Freedom last year, has recently voiced many concerns about the future of life on the earth. In April, he spoke about his fear of aliens in an interview for the Discovery Channel. "If aliens visited us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. "Such advanced aliens would perhaps become wanders, looking to conquer and colonize whatever planets they can reach."  But Hawking says he is still hopeful for the future of humanity. "I'm an optimist," he told Big Think. "We have made remarkable progress in the last hundred years. But if we want to continue beyond the next hundred years, our future is in space." Getting to another planet will prove a challenge, not to mention colonizing it for humanity. University of Michigan astrophysicist Katherine Freese told Big Think that "the nearest star to the earth is Proxima Centauri which is 4.2 light years away. That means, if you were travelling at the speed of light the whole time, it would take 4.2 years to get there--or about 50,000 years using current rocket science." Which of the following is NOT a reason why humans need to give up the earth according to the passage?
Lots of great dangers for humans have been seen. It will be hard for humans to avoid disasters on the earth in the future. The earth will be destroyed by worldwide wars before long. Advanced aliens may conquer and colonize the earth some day.