Task: Question Answering
Input (English): Passage: Solar energy may be used in a water stabilisation pond to treat waste water without chemicals or electricity. A further environmental advantage is that algae grow in such ponds and consume carbon dioxide in photosynthesis, although algae may produce toxic chemicals that make the water unusable. Question: What is a reason why the water from a water stabilisation pond may be unusable?
Output (English): algae may produce toxic chemicals