Organs I guess.
(Sorry if Im not helping)
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.
I think horseback riding and biking! Hope this helped! :)
A responsibility is that you can take care of the child and a challenge is having to always be with the kid