A certain drink of type A is prepared by mixing 4 parts milk with 3 parts fruit juice. Another drink of type B is prepared by mixing 4 parts of fruit juice and 3 parts of milk. How many liters of fruit juice must be added to 21 liters of drink A to convert it to drink B?  Choose one:
A) 3
B) 4
C) 5
D) 6
E) 7

Rationale: "in 21 liters of drink a , there are 12 liters of milk and 9 liters of juice . with 12 liters of milk , we need a total of 16 liters of juice to make drink b . we need to add 7 liters of juice . the answer is e ."
Answer: E