If excepts raise interest then the book sales will increase. If excerpts are enough to replace person's desire to read the book then it will reduce book sales (example: for a crime thriller, if the criminal identified in the excerpt, will you be interested to read the book).
reply to swagatlaxmi,
there are 2 things happening...
1. excerpt is getting published.
2. increase sale is happening.
in between missing is the asumption i.e., excerpt being published stimulates a desire to buy the book which is resulting in a increase in a sale.
so choice A looks like an assumption to me.this cant be the conclusion.
having said so,now 1 and 2 are premises and we have to draw a conclusion.
choice D looks the only alternative.