I would say that <u>a deadly water-borne microorganism in a city's water source</u> would most quickly devastate a human population, merely because a huge number of people uses this water for some kind of a purpose.
The answer is C. influenza, chicken pox, and colds because influenza, chicken pox, and colds (called rhinovirus) are all caused by viruses. Food poisoning is not a virus, athlete's foot is a fungal infection, and strep throat is a bacterial infection.
Any child born at 36 weeks or earlier is considered premature.