A relative pronoun introduces a relative (or dependent) clause in the sentence.
The relative pronouns who and whom give information about people.
To find out whether to use who or whom in a sentence, rewrite the dependent clause with a personal pronoun. If the clause uses a subject pronoun (he, she, or they), use who. If the clause uses an object pronoun (him, her, or them), use whom.
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The correct answer is
The smallest number is the one that comes first while counting.
To arrange the given numbers in order from smallest to greatest, find the smallest number among all the given numbers.
21 is the smallest number.