Regular aerobic exercise strengthens the heart, making it more efficient at pumping blood. When the heart muscle becomes fitter, each beat pushes more blood, so the heart can deliver more oxygen to working muscles during activity. Over time this increases cardiac output during exercise and lowers resting heart rate, improving overall circulation and oxygen delivery. That directly describes how the heart distributes oxygen to working muscles, which is the main cardiovascular benefit. Other statements don’t fit the cardiovascular focus: bones becoming denser relates to bone health, not the heart; lung capacity generally improves with training rather than decreases; and regular exercise supports immune function rather than compromising it.

Circuit training combines different fitness components in one session by moving through a series of stations, each with a different exercise. This setup lets you work strength, muscular endurance, and cardiovascular fitness all in the same workout, making it efficient and well-rounded. The approach is typically characterized by short transitions between stations to keep the heart rate elevated, which contrasts with traditional workouts that often focus on one component at a time with longer rests. It’s also versatile since it isn’t limited to free weights and can be arranged for groups, using a variety of equipment or bodyweight. This combination of training multiple components in a circuit is why it best describes circuit training.

Setting goals is a foundational step when building a physical activity plan. Goals give you a clear target and a timeline, guiding what activities to choose, how often to train, and how to progress. When goals are Specific, Measurable, Achievable, Relevant, and Time-bound, you can track progress, stay motivated, and adjust the plan as you improve. Without goals, it’s easy to drift, skip workouts, or lose direction. The other ideas don’t fit as well because they disrupt consistency or overlook essential elements. Randomizing workouts every week makes it hard to build a stable routine and achieve progressive overload. Ignoring your current fitness level can lead to choosing workouts that are too hard or too easy, increasing injury risk or causing stagnation. Focusing only on nutrition leaves out the exercise component, so you miss the benefits of training volume, intensity, and adaptation. A plan that starts with a clear goal and builds from your starting point is typically more effective and sustainable.

Two hours of recreational screen time per day is the recommended maximum for teenagers. This amount helps maintain a healthy balance between screen use and essential activities like sleep, physical activity, schoolwork, and face-to-face social interaction. Sticking to about two hours reduces the risk of sleep problems, less physical activity, and mood or attention issues that can come with excessive screen time, while still allowing teens to connect with friends and enjoy media in a moderation-friendly way. Spending more than two hours regularly can tip the balance toward sedentary behavior and other negative effects, and while one hour might seem safer, it can be too limiting for teens who rely on screens for communication and information.