That and which can both be used to introduce a relative clause that gives additional information about a place, thing, or idea.
The relative pronoun that introduces necessary information about things, people, places, and ideas. It introduces a restrictive clause. The meaning of the sentence would change if that clause is taken out.
The relative pronoun which introduces unnecessary information about things, places, and ideas. It introduces a nonessential clause. The meaning of the sentence does not change if that clause is taken out—the clause is providing extra information.
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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.