27. Let positive real numbers $x, y$ satisfy $x y=1$. Then the minimum value of $\frac{1}{x^{4}}+\frac{1}{4 y^{4}}$ is ( ).
(A) $\frac{1}{2}$
(B) $\frac{5}{8}$
(C) 1
(D) $\sqrt{2}$
27. C

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27. C