Let $$f(x)=\frac{x}{\sqrt{x^2-1}}$$. If $$f^2(x)=f(f(x))$$, $$f^3(x)=f(f^2(x))$$,...., $$f^n+1(x)=f(f^n(x)) $$, and so on, then $$f^{2019}(\sqrt{2})$$ is

A

1

B

0

C

$$\sqrt{2}$$

D

not define