Five points O,A,B,C,D are taken in order on a straight line with distances OA=a, OB=b, OC=c, and OD=d. P is a point on the line between B and C and such that AP:PD=BP:PC. Then OP equals:(A)a−b+c−db2−bc(B)a−b+c−dac−b(C)−a−b+c−dbd+c(D)a+b+c+dbc+ad(E)a+b+c+dac−bd