A researcher places a radio advertisement offering $32 in exchange for participation in a short study. The researcher accepts the first nine people who respond to the advertisement. Which of the following statements is true about the sample? It is a valid sample because the first nine people were selected to participate. It is not a valid sample because it is not a random sample of the population. It is a valid sample because money was offered to participants. It is not a valid sample because it is only a short study.