The velocity distribution in the boundary layer over a flat plate, set parallel to the direction of an incompressible free stream, is given by one-sixth power law of the form $\frac{u}{U}=\left ( \frac{y}{\delta } \right )^{\frac{1}{6}}$ . What is the ratio of displacement thickness to boundary layer thickness?
a. 1 : 8 b. 1 : 7
c. 7 : 72 d. 3 : 8