The cause of increased produce prices provides no info on whether those increased prices have, or have not, had an effect on gardening.

This says that 10 years ago, produce was cheaper and gardens were smaller. Now, produce is more expensive and gardens are larger. This strengthens the argument. We already had a correlation between rising prices and rising seed sales (at two specific companies, at least). Now we also have a direct correlation between rising prices and more gardening on average. This is correlation is great for the argument because it doesn’t require any link assumption between seed sales and gardening, and it also doesn’t rely on a small sample size.

Two problems. First, this is about community gardens, but the the argument is about personal gardens. Second, this suggests that interest in community gardening has increased, but offers no insight into whether the actual planting of such gardens has increased or whether produce prices are, or are not, the underlying cause.

Without knowing whether there’s been an economic downturn, we can’t assess the effect (D) has on the argument Also, even if there has been a downturn, produce prices could still be causing the rising popularity of gardens. Maybe, during economic downturns, produce prices increase, which in turn makes gardening more popular. So (D) doesn’t change the likelihood that higher prices cause more gardening. It has no effect on the argument.

This provides an alternative hypothesis for the increased seed sales at the two largest seed companies. The rise in sales has nothing to do with increasing produce prices or increased gardening activity. Those two companies are just scooping up customers from their out-of-business competitor.

This also suggests that maybe the two largest seed companies aren’t a representative sample of what’s actually going on in the seed industry more broadly.