A researcher posts an online advertisement offering $25 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? (1 point)

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 10 people were selected to participate.

d
It is not a valid sample because it is not a random sample of the population.