A hot sun warms the surface of a lake on a calm day. What will happen to the cold water at the bottom of the lake?

A: The cold water at the bottom will rise up and mix with the warm water at the top.
B: The warm water at the top will sink down and mix with the cold water at the bottom.
C: The cold water will remain at the bottom and won't mix very much with the warm water at the top.
D: Both A and B are true.