Echidnas are mammals that live in Australia and New Guinea． Scientists analysed the milk produced by female echidnas and identified a protein that they named EchAMP．The scientists predicted that EchAMP may have antibacterial properties． The scientists tested the effect of EchAMP on the bacterium Escherichia coli． 1． 100 E．coli cells were added to each well on a cell culture plate with 96 wells． 2．A treatment solution that contained EchAMP was added to each well on the plate． 3．A chemical that causes living E．coli cells to fluoresce was added to each well． 4．The plate was incubated at $$\(37^{\circ} \mathrm{C}\)$$ ． 5．Every hour for 7 hours，the fluorescence emitted by the E．coli on the plate was recorded as a measure of E．coli population growth． 6．Steps $$\(1-5\)$$ were repeated eight times． The scientists also carried out two control experiments． －A negative control experiment repeated the procedure（steps $$\(1-6\)$$ ），but the treatment solution did not contain EchAMP． －A positive control experiment repeated the procedure（steps 1－6），but the treatment solution contained an antibiotic called bacitracin instead of EchAMP． Explain why the scientists included the negative control experiment in their investigation. . . (ii) Explain why the scientists included the positive control experiment in their investigation. . .

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