Which class of organisms are likely to be categorized as pollution sensitive?

Pollution-sensitive organisms are those that can be greatly affected by changes in water quality, especially due to pollution. These organisms typically have less resilience to stressors such as toxins or nutrient overloads. Class A organisms in the context of water quality assessments usually includes species that are highly sensitive to pollutants, such as certain species of mayflies, stoneflies, and caddisflies. These organisms are often used as indicators of healthy aquatic ecosystems because their presence suggests good water quality.

In contrast, organisms categorized in other classes may not exhibit the same level of sensitivity to pollution. For instance, Class B organisms might be moderately sensitive, while Class C and Class D may include species that can thrive in more polluted environments, making them less useful as indicators of water quality. Thus, Class A represents a group that is particularly useful for monitoring changes in ecosystem health and assessing the impact of pollution, making it the correct answer in identifying pollution-sensitive organisms.