publishers don't see e-books as a threat to written books ( ),they see them as an opportunity to draw in a new generation of readers
a instead
b besides
为什么选a不是b呢，b也可以吧

publishers don't see e-books as a threat to written books ( ),they see them as an opportunity to draw in a new generation of readers
a instead
b besides
为什么选a不是b呢，b也可以吧

B不能用。
根据句子的意思来看，前后是表示相反（或者说是相互否定）的意思，所以应该用instead。而besides则表示的是前后两句之间具有相同意义或者说更进一步的意义。