请教高手——狒狒39

lxw19

39. Quasars---celestial objects so far away that their light takes at least 500 million years to reach Earth---have been seen since 1963. For anything that far away to appear from Earth the way quasars do, it would have to burn steadily at a rate that produces more light than 90 billion suns would produce. But nothing that burns at a rate that produces that much light could exist for more than about 100 million years.  



If the statements above are true, which one of the following must also be true on the basis of them?  



A.       Instruments in use before 1963 were not sensitive enough to permit quasars to be seen.  

B.       Light from quasars first began reaching Earth in 1963.  

C.       Anything that from Earth appears as bright as a quasar does must produce more light than would be produced by 90 billion suns.  

D.       Nothing that is as far from Earth as quasars are can continue to exist for more than about 100 million years.  

E.        No quasar that has ever been seen from Earth exists any longer.

答案选的是E，看解释没有看懂，请问这是为什么呢？

lxw19

哦，好像有些明白了，是不是说那个Q已经不在了，但是它在500 million years 以前发出的光还在向地球这个方向传播呢？

wooyeon

就是Q这种天体发出的光要500million years才能到达地球
但是Q这种天体只可能持续燃烧100million years 之后就没了
所以在地球上观测到的时候Q就已经not exist了