Indicators of fecal water pollution

Which of these is a bio indicator of past pollution with fecal matter?
a) Faecal Streptococci  b) Clostridium perfringens c) both  d) none

The answer is b.

How to remember:
Cl. perfringes is an indicator of past water pollution
Also remember that the indicator of recent water pollution is Faecal Streptococci ( Recent = Short term - Streptococci )