Answer:
By counting stars.
Explanation:
During the late eighteenth century, there was an astronomer - William Herschel who tried to estimate the size as well as shape of the galaxy by on the sky. It was the time when astronomers didn't even know that there exist some galaxies outside the milky way. He counted all the stars and created a map of the galaxy.
However, in that attempt he didn't realize that stars he was counting were not clear from his view. With that attempt of incorrect nature, he concluded his work as, the galaxy is disk-shaped, flat and contains sun at the center. Today, when the astronomers got a perfect idea of the vastness of the universe and galaxies, we are in a position to say that Herschel's idea of the galaxies was very tiny and underestimated.
Because it starts with "although", you realize there is a negative point coming in the second part of the sentence, which leads us to C and D. However, D is the one which is correct, because, in fact the genetic variation is diminished by the process of cloning. The new living organism created is a copy of some other. This way, it has the same DNA. If it the were a traditional case of reproduction, the new living organism would have a different DNA code, which leads us to answer D.
Viruses are consisted particles of genetic material (RNA or DNA) coated with a protective protein case that can cause infections. Viruses lack the necessary metabolic machinery to replicate its genetic material and produce proteins, so they infect host cells, highjack their cell mechanisms and use it to reproduce.
Answer:
they are able to perform a specialized function in the body
