Fatima wants to know how long the volunteers at an animal shelter have been working there. She plans to sample 30 of the shelter's 140 volunteers. Describe a way that Fatima could choose her sample so that it is likely to be representative of the population.
a) Randomly select volunteers from different age groups
b) Randomly select volunteers from different departments
c) Randomly select volunteers based on their years of service
d) Randomly select volunteers based on their gender