Most fish exchange gases using gills on either side of the pharynx (throat). Gills are tissues which consist of threadlike protein structures called filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia.
Synathropic is the term which is used to describe fly species which exploit foods and habitat created by agriculture and other human activities. They are considered as pest and also as vectors to diseases which live near human and benefit from human and their dwelling.
The answer is D) the organ system.
