The students only tested the drinking water supplied by the lake. This 80% may not contain the bacterial infection, but the other 20% from unknown sources(which they didn't account for in their conclusion) may.
d. Other sources of drinking water were not tested for bacteria ;
The flaw in the students' experimental design is that they only tested water from the lake, which supplies 80% of the town's drinking water, without examining other possible sources of drinking water. Testing only one source can lead to an incorrect conclusion that the drinking water is bacteria-free. A comprehensive approach should involve testing all potential water sources to draw a valid conclusion.
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