Which fact-finding method is a cost-effective way to collect information from many people simultaneously?

Explanation

Option A refers to observing behaviors directly, which can be time-consuming and costly. Option B, work sampling, involves intermittent observations and may not cover large groups efficiently. Option C, interviews, require significant time and resources when conducted with many individuals. Therefore, none of these methods are inexpensive for gathering data from a large audience, making option D the correct choice.