For an incompressible boundary layer flow over a flat plate shown in the figure, the momentum thickness is expressed as
- $\int_0^{\infty} \frac{u}{U_{\infty}} d y$
- $\int_0^{\infty}\left(1-\frac{u}{U_{\infty}}\right) d y$
- $\int_0^{\infty} \frac{u}{U_{\infty}}\left(1-\frac{u}{U_{\infty}}\right) d y$
- $\int_0^{\infty}\left(1-\frac{u^2}{U_{\infty}^2}\right) d y$
