If $$U_\infty$$ = free stream velocity, u = velocity at y, and $$\delat = boundary layer thickness, then in a boundary layer flow, the momentum thickness $$\theta$$ is given by

A

$$\theta = \int_{0}^{\delta} \frac {u}{U_\infty} \left( 1 - \frac {u}{U_\infty}\right) dy$$

B

$$\theta = \int_{0}^{\delta} \frac {u}{U_\infty} \left( 1 - \frac {u^2}{U_\infty ^2}\right) dy$$

C

$$\theta = \int_{0}^{\delta} \frac {u^2}{U_\infty^2} \left( 1 - \frac {u}{U_\infty}\right) dy$$

D

$$\theta = \int_{0}^{\delta} \left( 1 - \frac {u}{U_\infty}\right) dy$$