What type of detectors respond to the presence of smoke?

Detectors that respond to the presence of smoke include both photoelectric and ionization detectors, making the answer D the appropriate choice.

Photoelectric detectors work by using a light source and a sensor. When smoke enters the detection chamber, it scatters the light beam, which triggers the alarm. This type of detector is particularly effective at identifying smoldering fires that produce a lot of smoke before flames are visible.

Ionization detectors, on the other hand, contain a small amount of radioactive material that ionizes the air in the sensing chamber. When smoke particles enter the chamber, they disrupt the ionization process, which causes a decrease in current flow and triggers the alarm. Ionization detectors are generally more responsive to flaming fires that produce little smoke initially but can ignite quickly.

Since both types of detectors are designed to respond to smoke in different ways and are effective for distinct types of fires, recognizing that both photoelectric and ionization detectors serve this function helps ensure comprehensive fire safety coverage in various environments.