As in the previous question, an airline wants to do a customer survey in order to improve its service. For one month, it sends an email to a random sample of customers which flew with the airline on the previous day (no customer will be contacted more than once). Again, the email states that the airline would like the customer to fill out a 10 minute survey in order to help the airline improve its service, but this time it states in addition that every respondent will receive a gift card worth $100. What can we say about this sampling plan