Certain ball bearings are packed in boxes that hold 500. 25 boxes of ball bearings are then packed in each crate for shipping. To make sure that the ball bearings being prepared for shipping were being produced with the correct measurements, a supervisor decided to take a sample for quality-control purposes. He randomly picked one crate, opened it up, and then randomly chose two boxes of ball bearings from that crate. He measured all of the ball bearings fro the two boxes. This is an example of a:

convenience sample

systemic sample

cluster sample

stratified random sample