In developing countries, most sewage is: a. Discharged, untreated, into surface waters.
b. Handled via communal septic tanks.
c. Subjected to primary (physical) but not chemical treatment.
d. Subjected to both primary and secondary sewage treatment

a. Discharged, untreated, into surface waters.

Explanation:

Most of the waste water and sewage water is left untreated into the surrounding soil or water which is the main cause of pollution. Due to lack of drainage system or septic chemical treatment the sewage water drain from major cities is discharged into the surface water untreated. This water can deteriorate the quality of portable water and can affect the population of many aquatic species reducing the aquatic biodiversity. The sewage discharge consists of nitrogen and phosphorus like minerals which can result in algal bloom causing eutrophication.

using the scientific method.