The proportion of gold in an alloy is measured in carats:
Table 1 shows information about two gold rings, C and D, containing only gold and copper.
Complete Table 1 by finding the missing values represented by the question marks.
| Gold ring | Carat | Mass of gold in grams | Mass of copper in grams |
|---|---|---|---|
| C | ? | 6 | 18 |
| D | 9 | 9 | ? |