A subject (a noun or a pronoun with modifiers) is who or what the sentence is about.
A compound subject has more than one person, place, thing, or idea.
Karen and John are going to the beach.
This sentence is about Karen and John who are going to the beach. As two people are mentioned, so this sentence has a compound subject.
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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.