Heat Is Added to a Substance at a Constant Rate

Heat is added to a substance at a constant rate. The substance starts as a solid and is melted; the liquid is heated and vaporized; finally, the vapor is heated at constant volume. This process is shown in the graph. The specific heat at constant volume of the vapor can be found by

A) multiplying the length of B in seconds) by the rate at which heat is added, and dividing by the mass of the substance.
B) multiplying the length of D in seconds) by the rate at which heat is added, and dividing by the mass of the substance.
C) dividing the rate at which heat is added by the product of the slope of A and the mass of the substance.
D) dividing the rate at which heat is added by the product of the slope of C and the mass of the substance.
E) dividing the rate at which heat is added by the product of the slope of E and the mass of the substance.