Terrestrial radiation is maximum in-
**Core Concept**
Terrestrial radiation refers to the exposure to ionizing radiation from natural sources in the environment, such as radon gas, cosmic rays, and radiation from the earth's crust. This type of radiation is a major contributor to the background radiation that humans are exposed to.
**Why the Correct Answer is Right**
The correct answer is **B. Ground level**. This is because radon gas, a radioactive byproduct of uranium decay, is present in the soil and can seep into buildings, leading to high levels of radiation exposure at ground level. Radon is the leading cause of lung cancer due to exposure to ionizing radiation, and its concentration is typically highest near the ground.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because radiation levels decrease with altitude, not increase. While cosmic rays do contribute to radiation exposure, their effect is more pronounced at high altitudes rather than ground level.
**Option C:** This option is incorrect because radiation from the earth's crust is indeed a significant contributor to terrestrial radiation, but it is not the primary source at ground level. The crust itself does emit radiation, but it is not as concentrated as radon gas.
**Clinical Pearl / High-Yield Fact**
It's essential to note that radon exposure can be mitigated by using radon-resistant construction materials and maintaining good ventilation in buildings.
**Correct Answer:** B. Ground level.