Which of the following is equal to ~$x^{18}$~ for all positive values of x ?
~$x^{9}+x^{9}$~
~$\left ( x^{2} \right )^{9}$~
~$\left ( x^{9} \right )^{9}$~
~$\left ( x^{3} \right )^{15}$~
x+18
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