An idiom is a phrase that means something different from the words that make it up.
Usually, you cannot figure out what the phrases mean by looking at the meaning of each word. So, you have to remember them or figure out their meanings by looking at how they are used.
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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.