Exercise: letter grade

Write a function grade(score) that turns a score into a grade. It returns:

• "A" for 90 points or more;
• "B" for 75–89 points;
• "C" for 60–74 points;
• "F" for fewer than 60 points.

Examples:

• grade(90) → "A"
• grade(74) → "C"
• grade(59) → "F"

Constraints: score is an integer from 0 to 100 inclusive.

Do not add extra imports unless the task says so explicitly.