An airline would like to find out the quality of service provided by their staff on one of its flights. The airline posted a staff right after the plane landed and told her to pass a survey form to the twentieth customer coming out of the flight. Thereafter, she would pass a survey form to every twentieth customer who came out of the door. What kind of sampling method is this?

• Systematic sampling 
• Cluster sampling 
• None of the other options 
• Simple random sampling