Aresearcher posts a mobile advertisement offering $15 in exchange for participation in a short study. the researcher accepts the first 15 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 15 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.
pls