Modify the grade book code from Unit $3$ so that it uses $3$ custom functions to calculate the average grade $($arithmetic mean, not letter grade$),$ report the highest grade, and find the lowest grade entered. This version of the program does not need to use heap memory, though you are welcome to do so$.$ The program should allow the user to indicate when he or she is done entering grades $($since the user may not have grades to fill the whole array$).$ When the user is done entering grades, the program should print out the grades entered by the user. The program should also display the average grade, highest grade, and lowest grade.