6. (30 pts) Using plain and/or multiway if-else statements ONLY write a program Gust the main function, no header files) that outputs the grade of a student given an integer variable 'score. The program should read the student's score as a argument passed to main0. If the score is outside the range of 0 and 100, the program should output "Not a valid score" and exit. Otherwise it should output the string "Grade is X" where X is replaced by the student's letter grade according to the following rubric: 90score