Q5 - advice from my tutor if the explanation didn't click for you that was helpful for me:

Sometimes, one way to strengthen a question, is to block a weakening point or objection to the argument. An objection to the author's argument might have been, "but what if publishers start producing more electronic copies and print less books? How will the libraries be able to keep stock of printed books?" Here, answer choice B refutes this potential objection. So by thinking this way, and also eliminating the other answer choices by process of elimination, it can help you better understand why B is right. (at least for me this helps!)