已知点\(P\)在以\(F _{1}\)，\(F _{2}\)为左，右焦点的椭圆\(C\)：\( \dfrac {x^{2}}{2b^{2}}+ \dfrac {y^{2}}{b^{2}}=1(b > 0)\)上，在\(\triangle PF _{1} F _{2}\)中，若\(∠PF _{1} F _{2} =α\)，\(∠PF _{2} F _{1} =β\)，则\( \dfrac {\sin (α+β)}{\sin \alpha +\sin \beta } = (\:\:\:\:)\)
A. \( \dfrac {1}{2}\)
B. \( \dfrac { \sqrt {2}}{2}\) C. \( \dfrac { \sqrt {3}}{2}\) D. \( \sqrt {2}\)
