If your heart is not strong enough or efficient enough, it is difficult to
a. strech
b. lift weights
c. breath
d. perspire

  C.) Breathe

Explanation:   Your heart pumps oxygen, and pushes out carbon dioxide, so if your heart is not strong enough or efficient enough then it is more difficult to breathe.

because health skills are tools and strategies and also practical skills are also something you need to learn (like tools and strategies) and put it in reality.