A skimmer is a device used in wastewater treatment plants (WWTPs) to remove oils and greases from the wastewater. Both oils and greases are contaminants that can cause issues in the treatment process and can have negative impacts on the environment if discharged untreated.

- Oils:
Oils are hydrophobic substances that do not mix with water. They can enter wastewater streams from various sources such as industrial processes, food processing, and domestic activities. Oils can float on the surface of the wastewater and form a layer, which can interfere with the biological treatment processes in the WWTP. In addition, they can also clog pipes and equipment. A skimmer is designed to remove these floating oils by skimming them off the surface of the wastewater.

- Greases:
Greases are semi-solid or solid fats that are also hydrophobic and do not mix with water. They are commonly found in wastewater from food preparation and processing, as well as from certain industries. Like oils, greases can accumulate on the surface of the wastewater and impede the treatment process. A skimmer can effectively remove these greases by scraping or skimming them off the surface.

- Both of these:
Since a skimmer is specifically designed to remove floating oils and greases from wastewater, it can effectively remove both of these contaminants. By skimming off the surface layer, the skimmer collects and separates the oils and greases from the wastewater, allowing for their proper disposal or further treatment.

- None of these:
This option is incorrect because a skimmer is indeed used to remove oils and greases in WWTPs. It is a common and essential component of the treatment process to ensure the removal of these contaminants before the wastewater undergoes further treatment steps.

In conclusion, a skimmer in a WWTP is used to remove both oils and greases from wastewater. By effectively skimming these substances off the surface, the skimmer helps to prevent interference with the treatment process and promotes the overall efficiency of the wastewater treatment.