U.S Food and Drug Administration (FDA)
Answer:
The answer would be 4. just yourself
Explanation:
If you only have yourself for support then theres no system in place
Both Kyla and Tia will be in their zone. Even if they dicrease their heart rates by 20 beats per minute from 175 beats per minute. 155 beats per minute still falls under kyla's target heart rate zone and Tia's target heart rate zone.