已知数列\(\{a _{n} \}\)满足\(a _{1} = \dfrac {2}{3}\)，且对任意的正整数\(m\)，\(n\)，都有\(a _{m+n} =a _{m} \boldsymbol{⋅}a _{n}\)，则\(\{a _{n} \}\)前\(n\)项和\(S _{n}\)等于\((\:\:\:\:)\)
A. \(2-( \dfrac {2}{3})^{n-1}\)
B. \(2-( \dfrac {2}{3})^{n}\) C. \(2- \dfrac {2^{n}}{3^{n+1}}\) D. \(2- \dfrac {2^{n+1}}{3^{n}}\)
