Which is NOT a transmission method for flu viruses?

The correct answer is D, as outdoor temperatures do not serve as a transmission method for flu viruses. Instead, they can influence the spread of the virus in terms of environmental conditions, but they do not directly transmit the virus from one individual to another.

Transmission methods for flu viruses primarily occur through airborne droplets, which are released into the air when an infected person coughs, sneezes, or talks. These droplets can be inhaled by individuals nearby, leading to infection. Ingestion, while less common, could occur if someone comes into contact with contaminated surfaces or objects and then touches their mouth or food without washing their hands, facilitating the virus's entry into the body. Additionally, direct contact, such as touching or shaking hands with an infected person, can also lead to transmission if the virus is present on surfaces or the individuals themselves.

Thus, outdoor temperatures play a role in the epidemiology of flu, potentially affecting the virus's survivability and the immune response in populations, but they do not inherently function as a method for transmitting the virus between individuals.