Which one of the following is the expression for momentum thickness $\Theta$ of a boundary layer?
(a) $\Theta =\int_{0}^{\delta }\left [ 1-\frac{U}{U_{0}} \right ]dy$
(b) $\Theta =\int_{0}^{\delta }\left [ 1-\frac{U}{U_{0}} \right ]^{2}dy$
(c) $\Theta =\int_{0}^{\delta }\frac{U}{U_{0}}\left [ 1-\frac{U}{U_{0}} \right ]dy$
(d)$\Theta =\int_{0}^{\delta }\frac{U}{U_{0}}\left [ 1-\left ( \frac{U}{U_{0}} \right )^{2} \right ]$