**Core Concept**
Radiation therapy works by inducing damage to the DNA of rapidly dividing cells, ultimately leading to cell death. This is due to the ability of ionizing radiation to break DNA strands, causing double-strand breaks that are difficult for cells to repair.

**Why the Correct Answer is Right**
The destruction of rapidly growing cells by radiation therapy is primarily attributed to the induction of DNA damage through the formation of free radicals. When ionizing radiation interacts with water molecules in the cell, it produces free radicals such as hydroxyl radicals that can damage DNA. This damage, particularly double-strand breaks, triggers a cellular response that ultimately leads to cell death. The enzyme poly(ADP-ribose) polymerase (PARP) plays a key role in this response by facilitating the repair of DNA breaks, but its overactivation can also contribute to cell death.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while radiation therapy does cause DNA damage, it is not the primary mechanism by which it kills cells. DNA damage is a downstream effect of radiation-induced cell death.
**Option B:** This option is incorrect because while cell cycle checkpoints do play a role in responding to DNA damage, they are not the primary mechanism by which radiation therapy kills cells.
**Option C:** This option is incorrect because while apoptosis is a mechanism of cell death, it is not the primary mechanism by which radiation therapy kills cells.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that radiation therapy is most effective against rapidly dividing cells, which is why it's often used to treat cancers like leukemia and lymphoma.

**Correct Answer: C. Apoptosis is not the primary mechanism, but it can occur as a result of radiation-induced cell death.