If `|a|<1` and `|b|<1,` then the sum of the series `1+(1+a)b+(1+a+a^2)b^2+(1+a+a^2+a^3)b^3+...` is (a) `1/((1-a)(1-b))` (b). `1/((1-a)(1-a b))` (c.) `1/((1-b)(1-a b))` (d.) `1/((1-a)(1-b)(1-a b))`