Why Have Astronomers Come to Believe That Large Bodies of Liquid

Why have astronomers come to believe that large bodies of liquid water, if they exist on Mars, must be deep beneath the surface?

A) The Martian magnetic field appears to be generated by large bodies of water (containing dissolved minerals to make them electrical conductors) moving deep within the planet.
B) No signs of liquid water, past or present, have been found on the surface.
C) The surface of Mars does not buckle under the weight of its northern polar cap and is thus much thicker than previously thought.
D) The low water content of the basalt near Mars's large volcanoes suggests a deficit of water immediately below the surface.