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For what value(s) of $x$ is $f( x ) ={ x }^{ 5/3 }+{ x }^{ 1/3 }$ increasing the slowest?

$\left( \cfrac{1}{5}\right) ^{ 3/4 }$

$0$

$\pm \left( \cfrac{1}{5}\right) ^{ 3/4 }$

$\pm \left( \cfrac{1}{5}\right) ^{ 3/4 }$ and $0$