Briefly discuss what might account for this result and why it differs from the answer you gave to part $($a$).($Hint: Why was the finding of increased sales of physical books particularly strong for less popular books?$)$

A$.$ Digital and print book are always bought and read in tandem$,$ depending on the readers location and preference on a given day,

B$.$ Once someone reads a book they like in a digital version, they will always buy the print version to have as a back up$.$

C$.$ Unpopular books don't exist in physical form, so digital sales cause them to go into reprint and their demand to increase.

D$.$ As more books become available as free downloads, readers of these books recommend them to other readers, causing an increase in the demand for both the print and the digital versions of the book, particularly among readers who prefer to read a physical book.