If the lines $3x+4y-14=0$ and $6x+8y+7=0$ are both tangents to a circle, then its radius is

Question

$$\begin {array} {1 1} (a)\;7 & \quad (b)\;\frac{7}{2} \\ (c)\;\frac{7}{4} & \quad (d)\;\frac{7}{6} \end {array}$$