Passage Summary: Cities are cloudier on weekends than during the week. Since nature doesn't follow a weekly calendar, the only explanation left is that human behavior—which does follow a weekly calendar—is changing the weather.

Conclusion: Human activity has a significant, large-scale effect on the weather.

Reasoning: There is a measurable difference in weather between weekdays and weekends, and since nature does not have significant seven-day cycles, the cause must be human activity.

Analysis: The argument relies on a 'process of elimination.' It says 'It's not nature, so it must be humans.' For this to work, the argument must assume that the seven-day weather pattern isn't caused by something else we haven't considered. Specifically, it assumes that there isn't some other non-human cycle that happens to last seven days. Look for an answer that rules out other potential causes for a weekly weather rhythm, ensuring that human activity is the only remaining culprit.