$f(x) = (20 - x^4)^{1/4}$ for $0 < x < \sqrt{5}$, then $f(f(1/2))$ is equal to :

Question

( A ) $2^{-4}$
( B ) $2^{-2}$
( C ) $2^{-3}$
( D ) $2^{-1}$