The subject is who or what the sentence is about.
The predicate tells what the subject is doing. It is the verb along with some other words that tell more about the action done by the subject.
The predicate usually comes at the end of the sentence, after the subject.
The cat jumped over the wall.
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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.