How Is Water Ice Able to Remain on the Surface

How is water ice able to remain on the surface of Mercury, despite the planet's close proximity to the Sun?

A) The water ice is continuously replenished by condensation of water vapor from volcanoes.
B) The water ice is continuously replenished by fresh impacts from comets.
C) The water ice exists as permafrost below the thermally insulating surface and is exposed only by occasional impacts.
D) The water ice is permanently shielded from the Sun by crater walls at the north and south poles.