We use personal pronouns depending on the number:
Singular means one person.
Plural means a lot of people.
Use I when talking about yourself in 1st person.
Use you when talking about a friend or people.
Use he or she when talking about a friend or some other person.
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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.