Aresearcher 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.