Find the second derivative of f(x)= sq rt(x^2 + 1)

I think the first derivative is x(x^2 + 1)^-1/2

But, I don't know how to find the derivative of that?

Possible answers are:

A) x/sq rt (x^2 + 1)

B) sq rt(x^2 + 1) - 1/2(x^2 + 1)^-1/2

C) 2x(sq rt(x^2 + 1) + 1/2(x^2 + 1)^-1/2

D) 1/sq rt((x^2 + 1)^3)