已知函数\(f(x)\)满足：①对任意\(0\leqslant x _{1} < x _{2}\)，都有\( \dfrac {f(x_{1})-f(x_{2})}{x_{1}-x_{2}} < 0\)；②函数\(y=f(x+2)\)的图象关于点\((-2 , 0)\)对称．若实数\(a\)，\(b\)满足\(f(a ^{2} +2b)\leqslant -f(-b ^{2} -2a)\)，则当\(a∈[ \dfrac {1}{2},1]\)时，\( \dfrac {a}{a+b}\)的取值范围为\((\:\:\:\:)\)
A. \([ \dfrac {1}{8}, \dfrac {1}{2}]\)
B. \([ \dfrac {1}{4}, \dfrac {1}{2}]\) C. \([ \dfrac {1}{2},1]\) D. \([2 , 4]\)
