单项选择题

A.x=k1/r1+k2/r2……+kn/rn +kD
B.x=k1+r1+k2+r2……+kn+rn +kD
C.x=k1·r1+k2·r2……+kn·rn +kD
D.x=r1+r2……+rn +kD