Explain the chain of cause and effect reactions since the mid-1990s that led to lower book prices for american consumers.

The free enterprise system created many smaller booksellers who competed with the giant booksellers such as Borders and Barnes & Noble. 
Competiton from smaller booksellers in the mid-1990s is the cause that created an effect.

To remain competitive and profitable, the giant booksellers used their purchasing power to buy books in large volumes. Doing so enabled them to enjoy huge discounts and to pass on the savings to the customer.
The effect was to lower book prices for American consumers.

To summarize, competition in a free-enterprise system is the cause, and the lowering of book prices is the effect.

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