Aresearcher posts a newspaper advertisement offering $20 in exchange for participation in a short study. the researcher accepts the first five 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 only a short study.
b it is a valid sample because money was offered to participants.
c it is a valid sample because the first five people were selected to participate.
d it is not a valid sample because it is not a random sample of the population.