Answer:
Small sample size.
Step-by-step explanation:
The problem with Cliff's survey is that his sample size is too small. 10 out of 5,000 people represent only 0.2% of the population that attended the festival, which is too small of a sample for a reliable study.
Furthermore, since Cliff's question was to determine whether people, in general, prefer red apples or green apples, the sampling of only people who attended the festival might no be adequate.