*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 Smith chart is a graphical tool used in radio engineering to solve problems involving transmission lines and matching circuits. It represents complex impedances and admittances in a normalized form.

### Explanation:

1. **Correct Answer: The resistance axis**

   - **Resistance Axis**: On the Smith chart, the only straight line is the horizontal line through the center, known as the resistance axis. This line represents points where the reactance is zero, meaning the impedance is purely resistive. It runs from the left side (short circuit, $Z = 0$) to the right side (open circuit, $Z = \infty$).

2. **Why the Other Choices are Incorrect:**

   - **Reactance Axis**: The reactance axis is not a straight line on the Smith chart. Instead, it is represented by circles that are tangent to the resistance axis at the center of the chart. These circles indicate points of constant reactance.

   - **Current Axis**: There is no specific "current axis" on the Smith chart. The chart is primarily concerned with impedance and admittance, not directly with current.

   - **Voltage Axis**: Similarly, there is no "voltage axis" on the Smith chart. The chart does not directly represent voltage; it focuses on impedance and admittance.

### Key Points:

- **Resistance Axis**: Straight line, represents purely resistive impedances.
- **Reactance**: Represented by circles, not a straight line.
- **Current and Voltage Axes**: Not applicable to the Smith chart.

Understanding the layout of the Smith chart is crucial for interpreting impedance and admittance in RF circuits. The resistance axis is a fundamental reference line for these interpretations.