A popular soft drink company periodically sets up survey booths in various malls and grocery stores that are chosen at random. The employees working the booths ask random individuals to fill out a short survey on the quality of the company's drinks. While filling out the surveys, the individuals are told that a properly completed survey will be rewarded with gifts, such as free samples, T-shirts, generous coupons, and possibly an appearance in a new television commercial. What, if any, will be the most noticeable bias?
1) voluntary response bias
2) non-response bias
3) selection bias
4) response bias