A researcher 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.
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