Which of the following sampling procedures involve the use of probability sampling?

a. A college student is hired to interview a sample of voters in her town. She stays on campus and interviews 100 students in the cafeteria.

b. A pollster wants to interview 20 gas station managers in Baltimore. He posts a list of all such managers on his wall, closes his eyes, and tosses a dart at the list 20 times. He interviews the people whose names the dart hits.