Preview the following selection. Then read it and answer the questions.
Despite the proximity of the Moon to the Earth, the two bodies are glaringly dissimilar. The other three terrestrial planets bear some close similarities to either the Earth or the Moon, but again, each is unique, each teaching us something about the others and about ourselves.
Although Mercury, Venus, and Mars are all close to Earth, they were long shrouded in mystery. Venus is covered with clouds that perpetually hide its surface. Mercury has no significant atmosphere, so its rocky surface is accessible, but it is so close to the Sun that we can observe it effectively only in twilight, when our murky atmosphere provides a poor view. Mars, however, with its surface markings and polar caps, appears intriguingly Earthlike.
Before any physical analysis of the planets is possible, we need distances. The distances in AU between the bodies of the Solar System are found from their orbital locations. The distance in kilometers from the Earth to any of these bodies then gives the number of kilometers per AU (that is, the distance in kilometers between the Earth and Sun) and thus the distances in kilometers between all the planets. In practice, we use Venus, measuring its distance from Earth in kilometers by radar (radio direction and range).
(Taken from Astronomy! A Brief Edition, Kaler, James, 10th Ed,. Massachusetts: Addison Wesley, 1997, pgs 192-194)
What is the topic?