Given the following task, which would be the most appropriate method heading?

A method receives three whole numbers as input. The values represent grades. They should be unchangeable in the method. The method should return the average with a fractional component.

a. static double DetermineGrade(int grade1, int grade2, int grade3)

b. static int DetermineGrade(int grade1, int grade2, int grade3)

c. static int int int DetermineGrade(double finalAverage)

d. static double DetermineGrade(ref int grade1, ref int grade2, ref int grade3)

e. static void DetermineGrade( )