Awoman treated her home with a pesticide that kills spiders. the first application killed 78% of the spiders. two months later she applied the pesticide again, but it only killed 45% of the spiders. what would best explain the decrease in the effectiveness of the pesticide?

Natural selection. Only spiders that were more resistant to the pesticide lived through the first spraying of the pesticide, thus we're the only ones that could reproduce. This in turn made the offspring of the previous generation more resistant as well; thus making the pesticide less effective.