The best topic for the passage is "_________".

A．discover all stars and planets in the universe
B．gain more knowledge about the universe
C．study the craters, mountains and plains on the moon's surface
D．increase our knowledge of the earth

A．many more new groups of millions of stars remain unknown to us
B．stars are not in a single group
C．bigger and more powerful telescopes have been built recently
D．some stars are not single but in millions of groups