In the failure rate model, which is often represented as a 'bathtub curve,' there are three distinct periods: the early failure period (infant mortality), the normal life period, and the wear-out period. Section A represents the early failure or infant mortality period, where failures decrease over time as defective items are weeded out. Section B represents the normal life period with a relatively constant and low failure rate. Section C represents the wear-out period, where failures increase again due to ageing and wear-out of components. Catastrophic failures are most likely to occur during the wear-out period, which is represented by section C.