In an alloy of German silver, the ratio of copper to zinc was 21 : 16 while that of nickel to zinc was 7 : 24 What was the ratio of copper to zinc to nickel in the alloy?

• A. 21 : 16 : 7
• B. 17 : 21 : 4
• C. 63 : 48 : 14
• D. 68 : 28 : 21