1. It allows the scientist to determine the effect of the changed variable.
Let's say there was an experiment showing the effect of water on plant growth. The environment (in this case, the other variables including the amount of sunlight) would need to be kept under control in order for the results to be clear. If there was a different amount of sunlight during each trial, then the results would be effected by both the water and sunlight, making the results incorrect for the experiment.
Answer:
I think it's b,sorry if im wrong
D is the correct answer to this
The theory of evolution was created by Charles Darwin. He found that we evolve through natural selection. We survive and reproduce in order for our species to survive.