Which of these species is indicative of clean water?None of the answer alternatives.Sludge wormBloodwormStonefly nymph

In assessing the water quality of a particular environment, the presence of certain indicator species can provide valuable insights. Among the options you've presented, each species has different tolerance levels to pollution:

1. Sludge worm (Tubifex tubifex): This species is often found in polluted waters and can thrive in low-oxygen environments. They are not indicative of clean water.

2. Bloodworm (Chironomus spp.): Similar to sludge worms, bloodworms are tolerant of pollution and are commonly found in contaminated environments. Therefore, they are also not indicative of clean water.

3. Stonefly nymph (Plecoptera): Stonefly nymphs are sensitive to pollution and are typically found in clean, well-oxygenated streams and rivers. Their presence is a strong indicator of high water quality.

Given this analysis, the species that is indicative of clean water among the options is the Stonefly nymph.