Exercise: grade from a score

Write a function char grade(int score) that turns a score into a grade:

• 90 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.

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