What makes us think that Mars must once have had an atmosphere that was warmer and had higher surface pressure?
A) The fact that parts of Mars have a lot of craters tell us that Mars must once have been much warmer.
B) We think it for purely theoretical reasons, based on calculations showing that the Sun has brightened with time.
C) The atmosphere is too cold and thin for liquid water today, yet we see evidence that water flowed on the surface in the past.
D) The presence of inactive volcanoes on Mars tells us that there must once have been a lot of outgassing, and hence a thicker atmosphere.
Answer: C