Singular means one. Plural means two or more than two.
For most nouns, just add -s to make them plural.
one bag -- two bags
For nouns ending in s, x, z, ch or sh, add -es to make them plural.
one bus -- two buses
A singular noun takes a singular verb and a plural noun takes a plural verb.
Dan sings. [singular noun with a singular verb]
Kids sing. [plural noun with a plural verb]
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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.