It has been _________ years since Galileo made the first telescope.

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

A．What Is a Telescope
B．The History of Telescopes
C．How a Telescope Works
D．How Far a Telescope Can Reach