*Question T5B04:
Which is equal to one microvolt?*  

*Correct Answer is  A: One one-millionth of a volt*  

### AI Response: (may contain errors)  

To understand the concept of one microvolt, we need to break down the term "microvolt" and compare it to the other choices.

### Correct Answer: 
**One one-millionth of a volt**  
- The prefix "micro-" in the metric system denotes a factor of $10^{-6}$. Therefore, one microvolt (µV) is equal to:
  
$$
1 \, \text{µV} = 1 \times 10^{-6} \, \text{V} = \frac{1}{1,000,000} \, \text{V}
$$

This confirms that the correct answer is indeed one one-millionth of a volt.

### Why the Other Choices are Incorrect:
- **B: One million volts**  
  - This is incorrect because one million volts is represented as $1,000,000 \, \text{V}$ or $10^6 \, \text{V}$, which is much larger than one microvolt.

- **C: One thousand kilovolts**  
  - One kilovolt (kV) is $1,000 \, \text{V}$, so one thousand kilovolts would be:
  
$$
1,000 \, \text{kV} = 1,000 \times 1,000 \, \text{V} = 1,000,000 \, \text{V} = 10^6 \, \text{V}
$$
  - This is also much larger than one microvolt.

- **D: One one-thousandth of a volt**  
  - One one-thousandth of a volt is equal to one millivolt (mV), which is:
  
$$
1 \, \text{mV} = 1 \times 10^{-3} \, \text{V} = \frac{1}{1,000} \, \text{V}
$$
  - This is still significantly larger than one microvolt.

### Summary:
- **Microvolt (µV)** = $10^{-6}$ volts = one one-millionth of a volt.
- The other choices represent much larger voltages and do not match the definition of a microvolt. 

Understanding these prefixes and their values is crucial for the Extra Class exam and for practical applications in amateur radio.