Question

ozone - depleting chemicals
the ozone layer is a crucial part of the stratosphere. it shields earth from harmful ultraviolet radiations. certain chemicals released at the earths surface can migrate up into the atmosphere, posing a threat to the ozone layer. scientists have noted increased ultraviolet radiation, revealing ozone holes. research identified ozone - depleting chemicals like cfcs, hcfcs, and halons. these compounds often contain chlorine.
what reveals ozone holes?
lack of sunlight
increased birds in an area
increased ultraviolet radiation

Explanation:

The ozone layer blocks harmful ultraviolet radiation. When there are ozone - holes, more ultraviolet radiation reaches the Earth's surface. So increased ultraviolet radiation reveals ozone holes. Lack of sunlight is not related to ozone - hole detection and increased birds in an area has no connection to ozone holes.

Answer:

increased ultraviolet radiation