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