From a poll of people who recently bought cold medicine at all stores of a large drugstore chain, investigators concluded that the mean time between colds for all americans is 5.6 months.

C) Selection bias only

Explanation:

Selection bias in statistics is the result of not choosing the subjects randomly. If the subjects are not randomly chosen, then the sample cannot represent the entire population covered by the research.

In this case, only the customers of a certain large drugstore chain were surveyed, but what about the rest of the population? Unless someone was a client of that specific chain, he/she had no chance of being surveyed.