A beaker is filled with a liquid, 3 parts of which are water and 7 parts some medicine. What part of the mixture should be replaced with water so that that the resultant mixture has water and medicine in a ratio 1:1?

A

$$\frac{1}{7}$$

B

$$\frac{2}{7}$$

C

$$\frac{2}{5}$$

D

$$\frac{1}{5}$$