Assume That Variables Represent Positive Real Numbers $\sqrt { \sqrt [ 9 ] { y } }$

Assume that variables represent positive real numbers. Replace the radical with rational exponents
- $\sqrt { \sqrt [ 9 ] { y } }$

A) $\mathrm { y } ^ { 9 / 2 }$
B) $\mathrm { y } ^ { 9 }$
C) $y ^ { 2 / 9 }$
D) $\mathrm { y } ^ { 1 / 18 }$