Correct answer: (b) Only two
Explanation
- A
Only one
Only one. Nitrification in the biofilter is the core job; uneaten feed is mechanical, not biofilter.
- B
Only two
Only two. Biofilters convert ammonia to nitrate. They do not primarily remove uneaten feed, and they do not dose phosphorus for fish.
- C
All three
All three. Phosphorus is not increased as a nutrient service of the biofilter.
- D
None
None. Ammonia-to-nitrate is textbook RAS.
Summary. Official key is (b) only two. In a recirculating aquaculture system the biofilter is a nitrifying bed: ammonia → nitrite → nitrate. Solids / uneaten feed are settled or screened first. The unit does not add phosphorus as fish food.
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