Which of the following statements regarding the performance of concrete under fire conditions is MOST accurate?

A) Ordinary reinforced concrete is somewhat more vulnerable to failure than prestressed concrete systems.
B) The steel used to reinforce concrete must be protected to reach a fire rating equivalent to that required for the concrete.
C) Concrete structural systems can have a fire resistance rating from two to six hours.
D) Fire resistance ratings of a concrete assembly can be affected by concrete density and the load it must support.

D