The pronoun must be of the same number and person as its antecedent. An antecedent is a word that the pronoun refers to or replaces.
In case of relative pronouns, the verb shows the number and person of the relative pronoun.
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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.