Which statement best describes the relationship between heart rate and time during exercise? The heart rate increases for 6 minutes, remains constant for 19 minutes, and then gradually increases for 5 minutes. The heart rate decreases for 6 minutes, remains constant for 19 minutes, and then gradually increases for 5 minutes. The heart rate increases for 6 minutes, remains constant for 19 minutes, and then gradually decreases for 5 minutes. The heart rate remains constant for 6 minutes, increases for 19 minutes, and then gradually decreases for 5 minutes.