What is widely considered the most critical strategic error that led to the downfall of Borders Books?

In the competitive world of retail, strategic missteps can lead to the downfall of even the most established brands. Borders Books was once a dominant force in the bookselling industry, with hundreds of stores across the United States. Yet by 2011, this retail giant filed for bankruptcy and closed its doors forever. Test your knowledge about one of the most significant retail failures of the digital transition era and the critical business decisions that led to its demise.

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Outsourcing their online sales to Amazon.com instead of developing their own e-commerce platform
Expanding too aggressively into international markets, particularly in Asia and Europe
Investing heavily in music and movie merchandise while book sales declined
Refusing to carry e-readers of any kind in their stores until 2010