Let `A_r ,r=1,2,3, ` , be the points on the number line such that `O A_1,O A_2,O A_3dot` are in `G P ,` where `O` is the origin, and the common ratio of the `G P` be a positive proper fraction. Let `M ,` be the middle point of the line segment `A_r A_(r+1.)` Then the value of `sum_(r=1)^ooO M_r` is equal to
(a) `(O A_1(O S A_1-O A_2))/(2(O A_1+O A_2))` (b) `(O A_1(O A_2+O A_1))/(2(O A_1-O A_2)` (c)`(O A_1)/(2(O A_1-O A_2))` (d) `oo`