$\int _{ \dfrac { \pi }{4}}^{ \dfrac { \pi }{2}}d \theta \int _{0}^{ \cos \theta }r^{2} \cos \theta f(r)dr= \underline{\quad\quad\quad}$.
$ \text{A.}$ $\int _{0}^{1}xdx \int _{x}^{1}f(x,y)dy$
$ \text{B.}$ $\int _{0}^{1}xdx \int _{0}^{x}f(x,y)y$
$ \text{C.}$ $ \int _{0}^{1}xdx \int _{0}^{x}f( \sqrt {x^{2}+y^{2}})dy$
$ \text{D.}$ $ \int _{0}^{1}xdx \int _{x}^{1}f( \sqrt {x^{2}+y^{2}})dy$