What is life. B. The Archean environment. The uniqueness of liquid water on earth. Earth's size and distance from the sun make liquid water, and so life, on earth possible.
Earth is the third planet from the Sun.
This gives it a particular temperature range, roughly coinciding with the temperature at which liquid water exists. Mars is large enough to have once had volcanism, an atmosphere, and liquid water. But it cooled down and lost both volcanism and liquid water. Either from outgassing or accretion. Probably mostly from accretion because hot earth would have driven off early outgassing, and inner planets short on volatiles anyway.