设行列式 } 1 & 2 & a 2 & 0 & 3 3 & 6 & 9 = 3. 则 a = ( )A. (7)/(2)B. (5)/(2)C. -(5)/(2)D. 2

设行列式 $\begin{vmatrix} 1 & 2 & a \\ 2 & 0 & 3 \\ 3 & 6 & 9 \end{vmatrix}$, 余子式 $M_{21} = 3$. 则 $a = (\ )$

A. $\frac{7}{2}$

B. $\frac{5}{2}$

C. $-\frac{5}{2}$

D. $2$