A researcher posts a newspaper advertisement offering $10 in exchange for participation in a short study. The researcher accepts the first 10 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 is offered to participants.
c It is a valid sample because the first 10 people were selected to participate.
d It is not a valid sample because it is not a random sample of the population.