A researcher posts a radio advertisement offering $35 in exchange for participation in a short study. The researcher accepts the first eight people who respond to the advertisement. Which of the following statements is true about the sample? Select one:
a. It is a valid sample because the first eight people were selected to participate.
b. It is not a valid sample because it is not a random sample of the population.
c. It is a valid sample because money was offered to participants.
d. It is not a valid sample because it is only a short study.