PHET Energy Skate Park Simulation .Consider this equation: gravitational potential energy = mgh. Which of those variables is changing as the skater moves up and down the track?

Plzz hurry im being timed during times of vigorous physical activity, muscles require a constant supply of oxygen. how does the body respond to these needs? a. breathing rate and heart rate both decrease. b. breathing rate and heart rate both increase. c. heart rate decreases, but breathing rate remains stable. d. breathing rate increases, but heart rate remains stable.

Ineed to know why d is the correct answer asap

What is hydroelectric power produced by?