Catherine is an environmentalist studying water pollution in the stream shown in the diagram - Edexcel - GCSE Biology - Question 1 - 2011 - Paper 1

The location in the stream that is most polluted is C, as it has the least amount of freshwater shrimps found there, with only 1 shrimp recorded. Freshwater shrimps are considered indicator species because they can only survive in clean water. Therefore, their low numbers suggest that the water quality is poor at location C, indicating higher levels of pollution. In contrast, locations A and B both have 14 shrimps each, which suggests that these areas are less polluted.