JEE Math Practice Question

If $$\mathop {\lim }\limits_{n \to \infty } \,\,{{{1^a} + {2^a} + ...... + {n^a}} \over {{{(n + 1)}^{a - 1}}\left[ {\left( {na + 1} \right) + \left( {na + 2} \right) + ..... + \left( {na + n} \right)} \right]}} = {1 \over {60}}$$ for some positive real number a, then a is equal to :

1. A.7
2. B.8
3. C.$${{15} \over 2}$$
4. D.$${{17} \over 2}$$