Solve. -A Chemist Needs 7 Liters of a 50% Salt Solution

Solve.
-A chemist needs 7 liters of a 50% salt solution. All she has available is a 20% salt solution and a 70% salt solution. How much of each of the two solutions should she mix to obtain her desired solution?

A) 1.4 liters of the 20% solution; 5.6 liters of the 70% solution
B) 2.1 liters of the 20% solution; 4.9 liters of the 70% solution
C) 2.8 liters of the 20% solution; 4.2 liters of the 70% solution
D) 3.5 liters of the 20% solution; 3.5 liters of the 70% solution