*Question E9G07:
On the Smith chart shown in Figure E9-3, what is the only straight line shown?*  

*Correct Answer is  D: The resistance axis*  

### AI Response: (may contain errors)  

The correct answer to the question is **D: The resistance axis**. 

### Explanation:

1. **Why the correct answer is correct:**
   - The Smith chart is a graphical tool used in RF engineering to represent complex impedance. The resistance axis is the only straight line on the Smith chart, which runs horizontally across the chart. This axis represents pure resistive values, where the reactance is zero. 

2. **Why the other choices are incorrect:**
   - **A: The reactance axis**: This is not a straight line; instead, it consists of curved lines that represent different reactance values (inductive and capacitive).
   - **B: The current axis**: The Smith chart does not have a current axis. It primarily represents impedance and reflection coefficients, not current.
   - **C: The voltage axis**: Similar to the current axis, the Smith chart does not explicitly represent voltage. It focuses on impedance and reflection characteristics.

### Useful Concepts:
- The Smith chart is used to visualize impedance matching and to analyze transmission lines.
- The horizontal line (resistance axis) indicates where the reactance is zero, which is crucial for matching impedances in RF circuits.

Understanding the layout of the Smith chart and the significance of the resistance axis is essential for effective use in RF applications.