Task: Question Answering
Input (English): Problem: a tank is filled to one quarter of its capacity with a mixture consisting of water and sodium chloride . the proportion of sodium chloride in the tank is 50 % by volume and the capacity of the tank is 24 gallons . if the water evaporates from the tank at the rate of 0.5 gallons per hour , and the amount of sodium chloride stays the same , what will be the concentration of water in the mixture in 4 hours ? 
Options: a ) 40 % , b ) 35 % , c ) 30 % , d ) 25 % , e ) 20 %
Output (English): d