People who work with computers may, after many years, complain that their vision is getting worse. They blame the computers for their deteriorating sight. The real reason could be that vision normally deteriorates with age. The causal inference linking computer use with worsening vision could be wrong because the person's history of computer use and changes in their eyesight are... a- Systematically uncorrelated. b- Blinded. c- Confounded with their age. d- Randomly associated