\textbf{23 --} The integers from 9 to 19 are chosen at random. Two numbers are removed from these numbers and replaced by their difference. This process continues until all numbers are even, non-repeating, and the mean is as large as possible. What is the standard deviation of the new data?
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(1) $\sqrt{10}$ \hfill (2) $\sqrt{11}$ \hfill (3) $\sqrt{21}$ \hfill (4) $\sqrt{28}$
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