Natural radiation includes:
**Core Concept**
Natural radiation refers to the background radiation that originates from external sources, including cosmic radiation from space and terrestrial radiation from the Earth's crust. This type of radiation is a result of the decay of radioactive isotopes present in the environment.
**Why the Correct Answer is Right**
The correct answer is a type of natural radiation that is emitted from the Earth's crust. The Earth's crust contains small amounts of radioactive isotopes such as uranium, thorium, and potassium, which undergo radioactive decay and emit alpha, beta, and gamma radiation. This radiation is scattered throughout the environment and is a major contributor to the background radiation that we are exposed to every day.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a type of natural radiation. While radiation therapy is a medical treatment that uses ionizing radiation, it is not a natural source of radiation.
**Option B:** This option is incorrect because it is a type of artificial radiation. Nuclear reactors and nuclear power plants generate a significant amount of radiation, but this is not considered natural radiation.
**Option C:** This option is incorrect because it is a type of radiation that is emitted by the human body. While the human body does contain some radioactive isotopes, such as potassium-40, this is not a significant source of natural radiation.
**Clinical Pearl / High-Yield Fact**
It's worth noting that the average person is exposed to a significant amount of background radiation every day, including from natural sources such as cosmic radiation and terrestrial radiation. In fact, the average person receives about 2.4 millisieverts (mSv) of radiation per year from natural sources, which is a relatively small amount compared to the radiation we receive from medical imaging tests.
**Correct Answer: C. Cosmic radiation from the Earth's crust.**