Calcular:
$$ E = \begin{vmatrix} -\frac{1}{a} & \frac{1}{a+c} & \frac{1}{a+b} \\ \frac{1}{b+c} & -\frac{1}{b} & \frac{1}{a+b} \\ \frac{1}{b+c} & \frac{1}{a+c} & -\frac{1}{c} \end{vmatrix} $$

a) a      b) b      c) c      d) a+b+c      e) 0