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Which of these would NOT be equivalent to $\sqrt { y }$ ?

${ y }^{ \frac { 1 }{ 2 } }$

${ y }^{ \frac { 4 }{ 8 } }$

${ y }^{ \frac { 10 }{ 20 } }$

${ y }^{ \frac { 8 }{ 4 } }$