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

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.

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