Directions: Complete the passage with the best choice.

For centuries, scientists and philosophers have speculated on the structure of the universe and the existence of galaxies outside our own. But until the 20^th century, they were able to do no more than speculate. ______. Using grant telescopes, ultraviolet and infrared instruments, and high speed computers, they are studying the formation of galaxies for further insights into the nature of cosmos.

Meanwhile, the study of other galaxies has led to an extraordinary insight into the possible structure of the universe

We know, of course, that the universe is designed on a scale far more grand

After decades of speculation, in fact, we have started to penetrate to the very center of our own galaxy

Now, however, scientists have determined that the universe is made up of thousands of millions of galaxies

They have located regions that are very much disturbed with hot turbulent gases swirling at great force about the center