The layer of gold coating on a ring should be 0.03 millimeters thick when the ring is completed. A coating of 0.021 millimeters has already been applied. How many more millimeters of gold coating must be added to finish the ring?
A. 0.18 millimeters B. 0.009 millimeters C. 0.018 millimeters D. 0.09 millimeters