Every respondent that reaches the completion URL at the end of your survey will be rewarded. To avoid rewarding unqualified respondents, you'll want to screen them out by sending them back to Centiment using a different URL.
Set logic, screen-out URL, and hidden variables
Navigate to Skip/Disqualify Action on the last page of your survey just above your Thank You page
Select Edit and set your rules based on questions and responses
Scroll down the page, under If the Logic Passes, send the respondent to