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.