Heat Is Added to a Substance, but Its Temperature Does

Multiple Choice

Heat is added to a substance, but its temperature does not rise. Which one of the following statements provides the best explanation for this observation?

A) The substance must be a gas.
B) The substance must be a non-perfect solid.
C) The substance undergoes a change of phase.
D) The substance has unusual thermal properties.
E) The substance must be cooler than its environment.