1.设 $D=\left\{(x,y)|x²+y²≤4,x≥0,y≥0\right\}$, 且$a > 0$,$b > 0$,则$ I= \iint _{D} \dfrac {ae^{x^{2}}+be^{y^{2}}}{e^{x^{2}}+e^{y^{2}}}d \sigma =\underline{\quad\quad\quad}$.
$ \text{A.}$ $\frac {(a+b)}{4} \pi$
$ \text{B.}$ $\frac {(a+b)}{3} \pi$
$ \text{C.}$ $\frac {(a+b)}{2} \pi$
$ \text{D.}$ $(a+b)\pi$