The military wants to build a device for com-munication at ultrasonic frequencies.
At which of these frequencies should the device work to meet the military 's requirements ?

Options

A. 25 Hz
B. 220 Hz
C. 10116 Hz
D. 32842 Hz

Answer

To determine the frequency at which the military's communication device should operate to meet their requirement for ultrasonic frequencies, we can follow these steps:

### Step 1: Understand the Categories of Sound Frequencies
Sound frequencies can be classified into three main categories:
- **Infrasonic Frequencies**: Frequencies less than 20 Hz (not audible to humans).
- **Audible Frequencies**: Frequencies between 20 Hz and 20,000 Hz (audible to humans).
- **Ultrasonic Frequencies**: Frequencies greater than 20,000 Hz (not audible to humans).

### Step 2: Identify the Requirement
The military requires a device that operates at ultrasonic frequencies. This means the frequency must be greater than 20,000 Hz.

### Step 3: Evaluate the Given Options
We need to evaluate the provided frequency options to see which one falls into the ultrasonic category:
1. **25 Hz**: This is in the audible range (20 Hz to 20,000 Hz).
2. **220 Hz**: This is also in the audible range.
3. **10,116 Hz**: This is still within the audible range.
4. **32,842 Hz**: This frequency is greater than 20,000 Hz, placing it in the ultrasonic range.

### Step 4: Conclusion
Among the options provided, the only frequency that meets the requirement for ultrasonic communication is **32,842 Hz**.

### Final Answer
The device should work at **32,842 Hz** to meet the military's requirements for ultrasonic communication.

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