Write a "C" program for a professor to calculate the average grade of a student. The program should accept as many grades as the professor wants to enter. Therefore, ask the professor for the first grade, the second grade, and so on until the professor enters a negative value. A negative value indicates that the professor is finished entering grades. Once the professor is finished, your program should output: a) the average grade of the student b) the letter grade of the student

*for reference

( A grade is 90 to 100)

(B grade is 80 to 89)

(C grade is 70 to 79)

(F grade is 0 to 69)