Which phrase correctly describes the units Roentgen and milliRoentgen in relation to radiation measurement?

• A. Roentgen (R) is a unit used for high-range scales and milliRoentgen (mR) for low-range scales
• B. Roentgen (R) is a unit for energy and milliRoentgen (mR) for time
• C. Roentgen (R) and milliRoentgen (mR) are interchangeable
• D. Roentgen (R) is a unit for dose rate and milliRoentgen (mR) for dose

Answer: (A)

The key idea is that the Roentgen is a unit of exposure, a measure of ionization in air caused by X‑rays or gamma rays, and milliRoentgen is the same quantity divided by 1,000. In practice, detectors or readings that are small use milliRoentgens to keep numbers convenient, while larger exposures use Roentgens. So describing Roentgen as used for higher-range scales and milliRoentgen as for lower-range scales fits how these units are applied to readings. The other options mix up what the unit measures (exposure versus energy or time) or claim interchangeability or relate them to dose or dose rate, which isn’t correct for these units.