The answer to your problem is C. Respiration.
Strenuous muscular is just a fancy way to say exercise. As the muscles are used more often the need more oxygen, so that is why your breathing rate increases while doing "exercise".
Cons:
It kills animals
It destroys land and trees
It ruins animals homes
Pros
It clears up lane so we can build
Some rare animals need fire to reduce overhanging plants to live ( it’s called a karner blue Caterpillar btw)
And fire is a natural phenomenon that nature has evolved with