It is clear that human history will end the only mystery is when. It is also clear that if the timing is left to nature (or, if you prefer, to God) and humans hang on until the bloody end, the race's final exit will be ignoble (不光彩的). If future generations escape the saurian (蜥蜴类) agony of extinction by a wandering chunk of rock or ice, the sun's unavoidable growth to giant hood will still burn their last successors to ashes: only cinders and gases and dust will remain. Far future generations might prolong the process by posting colonies beyond the earth's orbit, but these would be sad outposts at the end of the solar system's long day, clutching memories of a lost planet and of billions of sacrificed souls. The difficulties—fantastic difficulties—of interstellar (的) travel might be overcome, but the mightiest of starships could do no more than defer the end of the world. An ignoble existence hopping from planet to planet—clinging to each clod until it, in its turn, was vaporized or frozen—might still be bearable were it not for the knowledge of its final uselessness. In the end, there is only death by gravity or entropy, the fiery quantum (量子) pit or the heatless grey soup. The great violinist Jascha Heifetz was great not least because he quit the concert stage at his peak, before the show became stale or the audience drifted away. To exit gracefully is sublime (美妙的), as Heifetz understood. And only one species is capable of choosing a similarly graceful exit all others march on like robots. To call time on the human race by choice, not necessity, would be the final victory of the human spirit over animal nature, an absolute emancipation from the command of DNA. Precisely because no other known life-form. could do or even conceive such a thing, humanity must. Science has revealed only one place in the universe that is hospitable to intelligent life, and humans are the only intelligence that, as far as is known, has ever enjoyed the opportunity to occupy it. If people left the stage after a reasonable run, in the fullness of time intelligence could evolve again (dolphin-people? Chimp--people? orchid(兰花)—people?). And then, in due course, when this new species deciphered (译解) human books or reached the marker that might be left for them on the windless moon, they would know that man ended his dominion so that theirs might begin. Imagine, then, how they will regard us. It is, far and away, the greatest act of goodness ever contemplated, the ennoblement of a whole species an act, almost, of angels. According to the passage, what might be human being's best choice for the final exit?