Write a program that allows the user to calculate the average grade for a class and displays the total number of each letter grade. The program should accomplish the tasks below:

1. The program will prompt the user to select an option to calculate or exit.
2. The program must ensure that the users input is valid, and notify the user to reenter for any invalid input.
3. The program must also allow the user to provide either uppercase or lowercase letter grades as input.
4. The program must use the following to calculate the class average: A=4.0, B=3.0, C=2.0, D=1.0, F=0
5. Submit a PDF document with screenshots of your program execution, ensure the screenshots show your programs execution for each of the tasks listed previously.
6. Submit your C source code to the assignment page on blackboard. Your submission should follow the formats provided 

//Sample Output
Welcome to the Gradebook, please enter an option:
   1. Calculate Class Average
   2. Quit
1

Please enter the letter grades for the class, to finish please enter Q
Enter a letter grade: A
Enter a letter grade: a
Enter a letter grade: b
Enter a letter grade: B
Enter a letter grade: d
Enter a letter grade: D
Enter a letter grade: c
Enter a letter grade: C
Enter a letter grade: f
Enter a letter grade: F
Enter a letter grade: Q

Calculated Entries:
There are 2 A's, 2 B's, 2 C's, 2 D's, and 2 F's
The class average is: 2.0

Please enter an option:
   1. Calculate Class Average
   2. Quit
2