The father of a junior high school student wants to determine the most popular book among junior high students. Select the sample with the least potential bias.
a. A randomly selected group of 10 book sellers
b. A randomly selected group of 30 junior high students.
c. A randomly delected group of 30 junior high school students leaving the public libary.
d. the group of 30 junior high students attending the birthday party of the researcher's child.