Josh works for MooMoo Milkshakes. The company wants to know what milkshake flavor is the most popular. Today, he surveyed every third female customer on their favorite milkshake flavor. Seventeen customers (out of 55 total) were surveyed, and 6 customers prefer MooChooChocolate, 5 customers prefer VeryStrawberry, and 6 customers prefer BananaBoBanna.

What type of sampling is demonstrated in the situation above?
random sampling
convenience sampling
negative sampling
census