$f(n)=\frac{1}{1+\sqrt2}+\frac{1}{\sqrt2+\sqrt3}+\frac{1}{\sqrt3+2}+...+\frac{1}{\sqrt{n-1}+\sqrt{n}}$

For $f(n)$ as defined above, which of the following options are correct?

- A: $f(10)=2$
- B: $f(15)=3$
- C: $f(20)=4$
- D: $f(25)=5$

Enter your answer as a 4-digit string of 1s and 9s $-$ 1 for correct option, 9 for wrong. For example: if A and B are correct, and C and D are incorrect, enter 1199. None, one or all may also be correct.