12.分式$\frac{1}{{a}^{2}-9}$,$\frac{2}{{a}^{2}+6a+9}$,$\frac{5}{a-3}$的最简公分母是( )
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| A. | (a+3)2(a-3) | B. | (a+3)2 | C. | (a+3)(a-3) | D. | (a-3)2(a+3)2 |