\textbf{138.} If one unit is added to all entries of the second column of matrix $A = \begin{bmatrix} 2 & 3 & 4 \\ 5 & a & 7 \\ 3 & b & 6 \end{bmatrix}$, what number is added to the value of the original determinant of the matrix?
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\hfill (1) $-3$ \hfill (2) $-2$ \hfill (3) $3$ \hfill (4) $6$ \hfill
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