*Question T6C05:
What is component 4 in figure T-1?*  

*Correct Answer is  D: Battery*  

### AI Response: (may contain errors)  

In question T6C05, we need to identify component 4 in figure T-1, which is indicated to be a battery.

### Explanation of the Correct Answer:
- **Correct Answer: Battery**
  - A battery is a common component in electronic circuits, providing the necessary voltage and current to power the circuit. It is typically represented in circuit diagrams with two parallel lines, one longer (positive terminal) and one shorter (negative terminal).

### Explanation of Incorrect Answers:
- **A: Resistor**
  - A resistor is used to limit current flow and is usually represented by a zigzag line or a rectangle in circuit diagrams. It does not match the typical representation of a battery.

- **B: Transistor**
  - A transistor is a semiconductor device used for amplification or switching. It is represented by a specific symbol that includes three terminals (collector, base, and emitter) and does not resemble a battery.

- **C: Ground symbol**
  - The ground symbol indicates a common return path for electric current and is usually depicted as a set of horizontal lines stacked on top of each other. This does not correspond to the appearance of a battery.

### Useful Rules of Thumb:
- **Identifying Components:**
  - Familiarize yourself with standard circuit symbols. Each component has a unique representation that helps in quickly identifying it in diagrams.
  
- **Battery Representation:**
  - Remember that batteries are depicted with two lines of different lengths, indicating the positive and negative terminals.

By understanding these symbols and their functions, you can effectively interpret circuit diagrams and answer related exam questions.