Because some bacteria is bad so it can cause illnesses and diseases and some are good.
Since a lot of people like to litter and not clean up around their community, it would attract rodents (mice, rats, raccoons). Also, bugs and insects (like cockroaches) would be attracted to the trash in the area. If there are rats then, something must eat them. This means that cats would be apart of this community. Humans naturally like to settle where water is so, fish would also be something to consider as your answer.
D - the endoplasmic reticulum (ER)
