The Northern Martian Polar Cap Decreases in Size During the Northern

The northern Martian polar cap decreases in size during the northern Martian summer but does not completely disappear. What is the explanation for this?

A) The polar cap is composed entirely of solid carbon dioxide (dry ice) .
B) The polar cap is composed partially of water ice.
C) Streams on the surface constantly keep the northern polar cap supplied with water during the summer.
D) The Martian atmosphere can hold only so much water vapor from the melting ice cap before the atmosphere becomes saturated.