Aresearcher posts a magazine advertisement offering $30 in exchange for participation in a short study. the researcher accepts the first three people who respond to the advertisement. which of the following statements is true about the sample? it is not a valid sample because it is not a random sample of the population. it is a valid sample because the first three people were selected to participate. it is a valid sample because money was offered to participants. it is not a valid sample because it is only a short study.