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 63 liters of drink A to convert it to drink B? Choose the correct choice from the following answers.
A) 7  B) 14  C) 21  D) 28  E) 35
In 63 liters of drink A, there are 36 liters of milk and 27 liters of juice.   With 36 liters of milk, we need a total of 48 liters of juice to make drink B.   We need to add 21 liters of juice.   The answer is 21.
<result>C</result>
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 63 liters of drink A to convert it to drink B?