The answer is: [C]: " -9 " .
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Note: The square root of a positive number will be a positive number. The negative value of a positive number will be a negative number only. That leaves us with "Answer choice: [C]: "-9" . Also, we know that the square root of 81 is "9" ; so the square root of "82" is close to "9" ; so the negative value of "-9" would be close to that value; so among the answer choices given, this value, "-9", makes sense.
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