the correct option would be a.
homologous structures are those structures which have a common structure and consequently origin (the bone structure is the same in both monkeys and bats in our case). these structures, however, perform different functions in different species (used for climbing in monkeys, and flight in bats).
they are called homologous structures because the underlying anatomy of these are same in different species, and indicative of a common ancestor. the difference in the appearance is due to the adaptation of these structures to accommodate different lifestyles of the species.
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the answer is c) gray water systems reduce the amount of fresh water used.