**Core Concept**
Ultrasound machines use high-frequency sound waves to produce images of the body's internal structures. The frequency of these sound waves is a critical parameter in medical sonography, as it affects the resolution and penetration of the ultrasound image.

**Why the Correct Answer is Right**
The correct answer is 2-10 MHz, which is the typical frequency range used in medical ultrasound machines. This frequency range allows for adequate penetration of sound waves through tissues while providing sufficient resolution for diagnostic imaging. The human eye can perceive sound frequencies up to 20 kHz, but medical ultrasound machines use frequencies above this range to produce images of internal structures. The specific frequency used depends on the application, with higher frequencies (e.g., 7-10 MHz) used for superficial structures and lower frequencies (e.g., 2-5 MHz) used for deeper structures.

**Why Each Wrong Option is Incorrect**
**Option A:** 20-50 kHz - This frequency range is too low for medical ultrasound imaging, as it would not provide sufficient resolution for diagnostic purposes.
**Option B:** 100-200 kHz - This frequency range is higher than typically used in medical ultrasound machines, which would result in reduced penetration and increased artifacts.
**Option C:** 1-2 MHz - This frequency range is too low for medical ultrasound imaging, as it would not provide sufficient resolution for diagnostic purposes.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the frequency range used in medical ultrasound imaging can vary depending on the application, and the choice of frequency affects the trade-off between resolution and penetration.

**Correct Answer: B. 100-200 kHz