B. One will better recognize more encompassing historical changes by studying entire periods rather than specific events. is your best answer
While sometimes, specific events can drastically change history, most of the time, it takes a build-up of events to change history, which means that it may take years or even centuries. For example, The fall of the Roman Empire was not as quickly as you would think. Civil war throughout the centuries, barbaric invasions, and diseases and failing economy throughout the years caused the Roman Empire to fall.
hope this helps