JEE Math Practice Question

Let S be the set of all the natural numbers, for which the line $${x \over a} + {y \over b} = 2$$ is a tangent to the curve $${\left( {{x \over a}} \right)^n} + {\left( {{y \over b}} \right)^n} = 2$$ at the point (a, b), ab $$\ne$$ 0. Then :

1. A.S = $$\phi$$
2. B.n(S) = 1
3. C.S = {2k : k $$\in$$ N}
4. D.S = N