Which of the following expression(s) is equivalent to $\frac{1}{27}$ ?  $I.$ $3^{-3}$  $II.$ $\left( \frac{1}{3} \right)^{3}$  $III.$ $\left( -\frac{1}{3} \right)^{3}$  $\textasciitilde$ 
选项为:A:$I$ only  B:$II$ only  C:$I$ and $II$  D:$I$, $II$, and $III$  请回答其中一个选项.