Let's break down each answer choice:

A. If many people who buy food in Arcadia County don't live there, this would suggest the buying habits aren't necessarily representative of the county's residents. But, this doesn't directly explain why more shoppers in the county are overweight.

B. Even if healthy-weight shoppers don't read nutritional labels, it doesn't clarify why the number of overweight shoppers has gone up. They could still be making healthy choices without reading labels.

C. Ah, this is interesting! If people cut down on saturated fat but end up consuming more calories overall, they could still gain weight. Excess caloric intake, regardless of the source, can lead to weight gain.

D. This contradicts the information given in the prompt. We know that the number of overweight shoppers has increased, so it can't be that the obesity rate has been decreasing.

E. Even if most fruits and vegetables from Arcadia County were shipped out of state, this doesn't specifically address why there's been an increase in overweight shoppers in the county.

Given our breakdown, the answer that best helps explain the increase in overweight shoppers in Arcadia County, despite the shift in food buying habits, is:

**C. People who reduce their intake of saturated fat often increase the number of calories they consume.**

This suggests that while people might be making healthier choices in terms of fat intake, they could be compensating by eating more calories overall, leading to weight gain. Hope that clears things up!