What quantity of water should taken out to concentrate 21 liters of 40% acidic liquid to 60% acidic liquid? Select:  A) 5 liters  B) 10 liters  C) 7 liters  D) 8 liters  E) 6 liters

Rationale: "required answer is = 21 ( 60 - 40 ) / 60 = 7 liters answer is c"
Answer: C