A professor gives 100-point exams that are graded on the scale 90-100: A, 80-89: B, 70-79:C, 60-69: D, <60: F. Write a Python function, grade(score), that takes a score as an input parameter and returns the corresponding letter grade. You are only allowed to use ONE if-statement with ONE elif or else for this problem. Write a main() function that prompts the user for a score, then calls your grade() function and finally prints the grade. Hint: think about how chr() could be used.