A low-pass filter with a roll-off rate of 60bB/decade is needed. The best combination is:
1 a 2pole filter followed by another 2-pole
2 two single-pole filters in series
3 a 2-pole filter followed by a 1-pole
4 none
1 a 2pole filter followed by another 2-pole
2 two single-pole filters in series
3 a 2-pole filter followed by a 1-pole
4 none