Question

one of the biggest sources of water pollution is______.
brushing your teeth
taking showers
runoff (agricultural, municipal, etc.)
swimming

Explanation:

Brushing teeth, taking showers and swimming are personal - use water activities that do not typically cause large - scale water pollution. Runoff from agricultural (pesticides, fertilizers) and municipal (sewage, waste) sources is a major contributor to water pollution.

Answer:

runoff (agricultural, municipal, etc.)