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Subtracting 1, 3, 5, ... from 81 reaches 0 at the 9th step. So the square root of 81 is:

A7
B6
C9
D8
Answer & Solution
Correct answer: C. 9
1. Each subtraction removes one odd number. 2. Nine steps reach zero. 3. So the root is 9. _Source: syllabus Mathematics, Chapter 6, Squares and Square Roots._
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