For a $$x \in R$$ (the set of all real numbers), $$a \neq -1$$,
$$\lim_{n \rightarrow \infty}\frac{(1^a + 2^a + ... + n^a)}{(n + 1)^{a - 1}[(na + 1) + (na + 2) + ... + (na + n)]}= \frac{1}{60}$$Then a =

A

5

B

7

C

$$\frac{-15}{2}$$

D

$$\frac{-17}{2}$$