Edward jenner's contribution for the eradication of small pox is

Options

A. his proposition that small pox had possibly spread throughout the world from indiaand China
B. his discovery of transformation procedure
C. his finding that rubbing of the skin crust of small pox victims on the arm of a healthy person , would develop resistance against small pox.
D. his finding that the cow pox infection protects the person from subsequent infection from small pox .