Xavier has been using a workout app to track his progress at the gym. Lately, he has found that his average heart rate during aerobic workouts is 129. His lowest target heart rate is 137. Which of the following will help him get the most productive workout, based on the information he has gathered?
a. Do anaerobic workouts only
b. Do fewer aerobic workouts
c. Increase the intensity of cardiovascular workouts
d. Decrease the intensity of cardiovascular workouts