A researcher posts a magazine advertisement offering $24 in exchange for participation in a short study. The researcher accepts the first six people who respond to the advertisement. Which of the following statements is true about the sample?

A. It is not a valid sample because it is not a random sample of the population.
B. It is a valid sample because the first six people were selected to participate.
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.