Answer:
Hi there!
The correct answer will be A. c+d = 400
.2c + .5d = .39
Explanation:
Since c and d represent the number of liters, and the lab requires 400 liters of solutions it make sense to have number of liters add up to 400.
Next, since they want it 39% acid solution, it makes sense to have percentages to add up to another percentage.