The correct answer is B. The key difference between skills sessions and drills is the training environment. Skills sessions provide crew members the opportunity to practice skills in a convenient, controlled training location, such as a classroom or simulator. In contrast, drills allow crew members to engage in practical exercises in a realistic setting, such as on the actual vessel or in a realistic scenario. The other options are incorrect because they either do not distinguish the training environments (A and C) or reverse the distinction between skills sessions and drills (D).