In a survey, respondents were asked to indicate their favorite brand of cereal (Post or Kellogg's). They were allowed only one choice. What is the probability concept that implies it is not possible for a single respondent to state both Post and Kellogg's to be the favorite cereal?

a. Concept of mutually exclusive events

b. Concept of independent events

c. Concept of dependent events

d. Concept of mutually inclusive events