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