Step-by-step Solution:

Correct Choice: C

Why C is the Best Fit

The study indicates that two-thirds of the children developed myopia after starting school, while their illiterate parents and grandparents (who had no formal schooling) showed no signs of it. The strongest and most direct inference is that an activity associated with school—most plausibly the visual stress from sustained near work like reading and classwork—is a contributing factor to the children’s nearsightedness.

Why the Other Options are Not Supported

• A. “Only people who have the opportunity for formal schooling develop nearsightedness.”
  This statement is too strong and universal. The study is limited to one area and cannot be used to rule out other causes of myopia in non-schooled people elsewhere in the world.
• B. “People who are illiterate do not suffer from nearsightedness.”
  This is also a universal overgeneralization. While the parents and grandparents in this specific sample showed no signs, that doesn’t prove that all illiterate people are free from nearsightedness.
• D. “Only literate people are nearsighted.”
  This makes the same overreaching claim as options A and B. The data from this single study do not justify such an exclusive and universal conclusion.

Therefore, option C is the most reasonably and strongly supported conclusion based on the provided text.