Exercise: letter grade

Write a function function grade(score) that turns points into a letter grade:

• 90 points or more: "A";
• 75–89: "B";
• 60–74: "C";
• below 60: "F".

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.