Final answer:
For vigorous-intensity aerobic activity in a limited time, Nadine can opt for dancing or bicycling uphill, which are efficient for cardiovascular fitness and can fit into her busy schedule. Hence, option (a) and (c) are correct.
Step-by-step explanation:
Nadine is looking to maintain her fitness but has limited time due to long work hours. To save time and still achieve vigorous-intensity aerobic activity, she might choose options such as dancing or bicycling uphill. These activities are efficient for cardiovascular fitness and can be incorporated into a shorter duration with higher intensity to fit into a busy schedule. Other options like sit-ups and squats are generally considered anaerobic exercises and brisk walking, while beneficial, is typical of moderate intensity.
Aerobic exercise, like bicycling uphill, will ensure that Nadine maximizes her workout time as she uses large muscle groups, which leads to increased heart rate and improved cardiovascular endurance. Dancing is also a great alternative because it is a fun aerobic activity that can lead to high energy expenditure, effectively combats stress, and improves cardiovascular health. Remember, it is essential to find activities that are enjoyable to ensure long-term adherence to a fitness routine.