Two circles of radii `r_1 and r_2 (r_1 >r_2)` touch each other externally. Then the radius of circle which touches both of them externally and also their direct common tangent is (A) `(r_1r_2)/(sqrtr_1+sqrtr_2)^2` (B) `sqrt(r_1r_2)` (C) `(r_1+r_2)/2` (D) `(r_1-r_2)/2`