The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason

(a)

Ice is a bad conductor of heat.

(b)

Since the surface of the lake is at the same temperature as the air, no heat is lost.

(c)

The density of water is maximum at 4°c.

(d)

None of the statements (a), (b) and (c) given is correct.