Background Paragraph: Retinitis is inflammation of the retina in the eye, which can permanently damage the retina and lead to blindness. The retina is the part of your eye that is also known as the "sensing tissue." Retinitis may be caused by a number of different infectious agents. Retinitis, also called Retinitis pigmentosa, has a prevalence of one in every 2,500-7,000 people. This condition is one of the leading causes that leads to blindness in patients in the age range of 20–60 years old. 
Story: Local hospital has launched a program to check if a patient has Retinitis. Resident ophthalmologist David is in charge of this program. In the first day of the program he checked two patients, patient A and patient B. The test result showed that patient A had Retinitis, but patient B did not have Retinitis. Then next day 
David checked two other patients, patient C and patient D. Patient C was 40 years old, but patient D was 80 years old. He found both patient C and D had Retinitis.
For which patient Retinitis will be one of the leading causes of blindness, patient C or patient D? For which patient Retinitis will not be one of the leading causes of blindness, patient C or patient D? Which patient is in risk of becoming blind, patient A or patient B? Which patient is not in risk of becoming blind, patient A or patient B? Which patient would have inflammation of retina, patient A or patient B? Which patient would not have inflammation of retina, patient A or patient B? Will David find patient A is in risk of blindness or not in risk of blindness? Will David find patient B is in risk of blindness or not in risk of blindness? Will patient A have inflammation or no inflammation of retina? Will patient B have inflammation or no inflammation of retina?