amitaryaWe don't really have enough information to assess the chronological order of any of these events. It's possible, for instance, that fantasy movies have been gaining in popularity every year for a decade or more. Perhaps a few years ago, libraries responded by stocking more fantasy books, and this year it finally took a toll on book sales. That would be a plausible alternative to the author's causal story.
Certainly, it's also possible that the rise of fantasy movies led to two simultaneous results: 1) fans started skipping the books in favor of movies and 2) libraries decided to stock more (possibly unwanted) fantasy books. If this were the case, we would NOT have an alternative to the author's conclusion.
So how do we decide if D really weakens? Simple--we acknowledge that we don't know how it all happened. All we know is that there is another phenomenon (library stock) that MAY have caused the drop in sales. That's all we need--a weaken will almost never actually disprove the conclusion.