$f(x)=\large\frac{\log(1+ax)-\log(1-bx)}{x}$ is not defined at $x=0$ .The value which should be assigned to f at $x=0$.So that it is continuous at $x=0$ is

Question

$(a)\;a-b\qquad(b)\;a+b\qquad(c)\;\log a+\log b\qquad(d)\;\large\frac{a+b}{2}$