Aresearcher 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? 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.