Water supplies are treated with chlorine to kill pathogens. Chlorine reacts well with many other chemicals and bacterial cells because it is a strong oxidant. However, Legionella pneumophila can sometimes evade killing by residual chlorine. How does it do this

By forming Biofilms

Explanation:

Legionella pneumophila forms biofilms by using the residual chlorine and hence act as killer of other microbial cells and intracellular pathogens.  It also colonizes within multispecies microbial communities and kills them

specific gravity is the density of a substance divided by the density of water. since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density

so "c" is the answer

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