Answer:
The correct answer is air resistance.
Step-by-step explanation:
Isaac Newton who was the one who proposed the law of gravity, established that two bodies that are thrown, fall down at the same time regardless of their weight, but this only works if the bodies are thrown into a vacuum.
It was Gelileo Galilei who demonstrated that if the bodies are intervened by air resistance, they will not fall at the same time. That is, because of air resistance, if a brick and a feather fall through the air, the brick will fall faster, and the feather will do so more slowly.