Which 19th century literary subscription innovation made books widely affordable to the middle class for the first time?

Before digital subscription services like Kindle Unlimited or Audible transformed how we consume literature, there was an innovative subscription model in the 19th century that revolutionized book access for the average person. This poll tests your knowledge of historical innovations in literary distribution that shaped modern publishing and reading habits.

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Circulating libraries with annual membership fees
Serial publication, where novels were released in affordable monthly installments
Book clubs with quarterly selections sent by mail
Newspaper supplements with condensed classic novels