The questions in this group are based on t

题目：

The questions in this group are based on the content of a passage. After reading the passage, choose the best answer to each question, Answer all questions following the passage on the basis of what is stated or implied in the passage.

Forget hostile aliens. According to a forthcoming book by noted astrophysicist Egbert Larson, the intrepid humans who first attempt interstellar space travel will face far more daunting challenges before they ever meet the Little Green Men.

Larson begins with the problem of relativistic time dilation. If you travel all the way to Alpha Centauri, you’d like to come back and tell your friends about it, right It’s not too likely to happen, though. If Mr. Einstein was right about relativity--and we’re not going to say he wasn’t--then time slows down when you approach the speed of light. A person traveling at any velocity near the speed of light will age only days for every week, month, or even year that passes on earth. Relativity does not present a problem for interstellar space travel, per se, but it does mean that interstellar civilizations or even just interstellar communications will require a mind-boggling amount of calendar juggling.

Did we mention that you’d have to travel at near the speed of light That’s because the distance between stars is so vast that even if you could travel at the speed of light--which, Larson reminds us, you can’t--it would take more than four years to reach our closest star neighbors, Alpha Proxima and Alpha Centauri, and decades or centuries to reach the other stars in our "immediate neighborhood." And if you tried to accelerate directly to the speed of light like they do in the movies, you’d be instantly splattered on the back of your theoretical spacecraft. Achieving anything close to light speed will require sustained accelera- tion at a level that human bodies can withstand--say, a crushing two gravities--for over a year. Better hope somebody brings some chips.

Speaking of chips, food is going to be a problem. Since it is economically, if not physically, impossible to accelerate 200 years’ worth of food to nearly the speed of light, and since you’re not likely to find any grocery stores along the way, someone will have to figure out how to make food in space. Keeping a crew alive on the way turns out to be the trickiest part of all. Once you’ve got the nearly impossible physics of space travel worked out, you still have to figure out the chemistry and biology of keeping your air and water clean and keeping your crew fed and safe from radiation and infection, and--did we mention the 200 years--you’ll probably need several generations of crew members to complete the trip. Ever been on a bus for more than 24 hours It’s not a pretty picture.

We applaud Larson for his insightful writing and his scrupulous attention to scientific detail. For those of you seeking a cold, hard look at the reality of interstellar space travel, this is a stellar read. But be warned: Larson doesn’t let you down gently. For those of you sincerely hoping to beam up with Scotty--and you know who you are--you might want to give this one a pass.

The passage implies that all except which of the following could be threats to human health during extended interstellar voyages ？（）

A. Meteor impact

B. Radiation poisoning

C. Accelerating too fast

D. Starvation

E. Old age