If $\int \frac{2 x^2+3}{\left(x^2-1\right)\left(x^2-4\right)} \mathrm{d} x=\log \left[\left(\frac{x-2}{x+2}\right)^{\mathrm{a}} \cdot\left(\frac{x+1}{x-1}\right)^{\mathrm{b}}\right]+\mathrm{c}$, (where c is the constant of integration) then the value of $a+b$ is equal to
- A$\frac{1}{12}$
- B$\frac{21}{12}$
- C$\frac{-1}{12}$
- D$\frac{-21}{12}$
✓ Correct answer: B
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