A barrel contains a mixture of wine and water in the ratio 3 : 1. How much fraction of the mixture must be drawn off and substituted by water so that the ratio of wine and water in the resultant mixture becomes 1 : 1 = ?

• A. $\frac{1}{4}$$
• B. $\frac{1}{3}$$
• C. $\frac{3}{4}$$
• D. $\frac{2}{3}$$