The function $f(x)=\large\frac{ln(1+ax)-ln(1-bx)}{x}$ 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

$(a)\;a-b\qquad(b)\;a+b\qquad(c)\;ln a-ln b\qquad(d)\;None\;of\;these$