This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Which of the following is not true about water quality estimation – 
Select and explain. 
A. Clostridium spores indicate recent contamination, B. Not more than 5% samples should have coliforms, C. Sodium thiosulfate added to neutralize chlorine, D. Coliforms must not be detectable in 100ml sample
A: Clostridium spores indicate recent contamination
Answer: A: Clostridium spores indicate recent contamination 
Explanation: Option 'a'

Coliform organism → Most commonly used bacteriological indicates
Faecal streptococci → Recent faecal pollution of water.
Spores of Cl. perfringence (in the absence of coliform group) → Remote faecal contamination of water.

About other options
Option 'b. & d'

Total coliform bacteria must not be detectable in any 100 ml of sample.
But, In case of large supplies, where sufficient samples are examined, coliform bacteria must not be present in 95% of samples taken throughout any 12 months period → Not more than 5% samples should have Coliforms.

Option 'c'

Sodium thiosulfate is used to dechlorinate tap water for aquariums or treat effluent from waste water treatments prior to release into rivers.
It is also used to lower chlorine levels in swimming pools and spas following superchlorination.