88. 令 $f(n) = \sum_{k=1}^{n-1} \frac{k^2}{n^3}$,求 $\lim_{n \to \infty} 8f(2n) = ?$(A) \(\frac{1}{6}\)(B) \(\frac{5}{6}\)(C) \(\frac{5}{3}\)(D) \(\frac{8}{3}\)