What is the part of the eye that focuses the image

Sadie and her mom were at a hardware store to purchase smoke detectors for their new home. sadie's mom was unsure how many units she should purchase. what is the general recommendation? a. one large one for the middle of the house b. every room in the house should have one c. kitchen, garage and laundry room- since those are where most fires happen d. every living area including the hallway by the bedrooms should have a smoke detector.