State police periodically set up roadblocks to check cars for inspection, insurance and drunk drivers. the state police would like to estimate how many cars in the city fail to have current inspections. the police choose 10 out of the 50 intersections of the city and checks inspection on every car that passes through the intersection. this is an example of

select one:
a. a cluster sample.
b. a systematic sample.
c. a simple random sample (srs).
d. a stratified sample.