Irradiation can indirectly form radiolytic products/free radicals from water.

• A. Irradiation can directly repair DNA.
• B. Irradiation can only kill cells by heating.
• C. Irradiation can indirectly form radiolytic products/free radicals from water.
• D. Irradiation has no chemical effects.

Answer: (C)

Ionizing irradiation commonly causes damage through indirect chemical effects by radiolysis of water. When radiation interacts with water in cells, it generates reactive species such as hydroxyl radicals, hydrated electrons, hydrogen atoms, and hydrogen peroxide. These radiolytic products are highly reactive and readily attack DNA and other biomolecules, producing strand breaks and base damage even when the radiation doesn’t directly ionize those targets. This indirect pathway is a major mechanism of radiation-induced cell killing, whereas claiming that irradiation only works by heating or that it has no chemical effects isn’t accurate. Direct DNA damage from radiation can occur, but the prevailing and most impactful mechanism in many contexts is the chemical damage driven by these water-derived radicals.