Researchers are curious if there is a connection between childhood cancer rates and living near power lines.
What could be a viable way to investigate this relationship?
I. Randomly select $200$ children and move half of them near power lines while ensuring the other half do not live near power lines then compare cancer rates.
II. Randomly select $100$ children who live near power lines and another $100$ children who do not live near power lines then investigate cancer rates of the respective groups.
III. Randomly select $100$ children who have cancer and another $100$ children who do not have cancer and compare where the children live.