A relative pronoun introduces a relative (or dependent) clause in the sentence. The five main relative pronouns are who, whom, whose, which, and that.
The relative pronoun whom gives information about people.
The relative pronoun that gives necessary information about things, people, and ideas. It introduces a restrictive clause. The meaning of the sentence would change if that clause is taken out.
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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.