All the Following Statements About Mars Are True

All the following statements about Mars are true. Which one might have led to a significant loss of atmospheric gas to space?

A) Mars lost any global magnetic field that it may once have had.
B) The axis tilt of Mars is thought to change significantly with time.
C) Outgassed water molecules are split apart, and the oxygen then reacts chemically with surface rock on Mars.
D) Mars probably once had a much higher density of greenhouse gases in its atmosphere than it does today.