Answer B

 Unless you're  a big fan of sci-fi, even the hint may not be enough . The answer is in Book One, and it's called the Law of Equilibrium. A scientist recently  published a paper using complex mathematical equations supporting  time travel and explaining away this paradox. But can we get there without such expertise? In the past, the analysis focused upon this paradox and argued, therefore, time travel would not be logically possible--at least not in a backward direct. 

Let's redefine the problem--easy now that someone has shown that time travel is possible. (Please note, I was lecturing on this many years before that paper came out.) Let us now ask how is it possible to avoid the time paradox when someone DOES go back in time? Books and movies have invented all sorts of work-a-rounds or special fictional rules for avoiding the paradox. Maybe we can learn something from all that fictional guess work.

 So having redefined our problem,  how do people avoid the potential paradoxes when time traveling? They don't. Even in the fictional accounts, avoidance by people does not work well or often. What's left? The Universe itself prohibits the paradox. Book One proposes two methods for the Universe to deal with potential paradoxes: The Law Of Equilibrium and, a very speculative, alternate Time-Path. That scientific paper mentioned above suggested the existence of what I have labeled The Law of Equilibrium. Simply stated, the Universe will neutralize or absorb any action or event that poses a paradox. (See Book One for more specifics.)

The Bottom line is that by redefining the problem and then looking at it from a different point of view even non-experts can develop possible solutions to previously impossible problems.