Which groundbreaking feature did Amazon first introduce in its 2007 Kindle that helped it succeed where previous e-readers had failed in the literary marketplace?

The publishing industry has seen remarkable technological innovations in the past few decades that have transformed how we consume literature. E-readers represented a revolutionary product innovation that changed reading habits worldwide. Test your knowledge about the development and impact of these digital reading devices in the literary marketplace.

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Wireless connectivity allowing direct book purchases without a computer
Color display technology
Social reading features that allowed readers to share annotations
Voice reading capability