Compound nouns are nouns made up of two or more words.
Some are just two words combined together to make a new word, while others are two words joined by a hyphen (-), or two words combined together to make a new word without being joined.
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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.