(a) Because the Earth is the only planet which has water and 70% if its surface is covered by water.
(b) Earth has the most ideal temperature conditions because it is neither too close nor too distant from the sun. This position in the Solar System makes it possible to receive neither excessive nor totally deficient heat from the Sun. Its average temperature surface temperature is 17 degrees Celsius far more favourable for living conditions than about 500°C on the nearest planets and about 150° to 200° Celsius below freezing point on the sunlit sides of distant planets. Earth is, thus, neither a burning furnace nor an extremely cold planet.
(c) Earth and its Moon are called a double planet because they revolve round the sun nearly in one and the same orbit.
