*Question E6C10:
In Figure E6-3, which is the schematic symbol for a NOR gate?*  

*Correct Answer is  D: 4*  

### AI Response: (may contain errors)  

In digital electronics, a NOR gate is a fundamental logic gate that outputs true or high only when all its inputs are false or low. The NOR gate is a combination of an OR gate followed by a NOT gate.

### Explanation:

1. **Correct Answer: D (4)**
   - The schematic symbol for a NOR gate is similar to an OR gate but with a small circle, known as an inversion bubble, at the output. This circle indicates the NOT operation, which inverts the output of the OR gate.
   - In Figure E6-3, symbol 4 has the shape of an OR gate with the inversion bubble at the output, correctly representing a NOR gate.

2. **Why the Other Choices are Incorrect:**
   - **Choice A (1):** This symbol represents an AND gate, which outputs true only when all inputs are true. It does not have the characteristic shape or inversion bubble of a NOR gate.
   - **Choice B (2):** This symbol represents an OR gate, which outputs true when at least one input is true. It lacks the inversion bubble that would make it a NOR gate.
   - **Choice C (3):** This symbol represents a NOT gate, which inverts a single input. It is not a combination of OR and NOT, as required for a NOR gate.

### Useful Rule of Thumb:

- **NOR Gate Logic:**
  - The output $Q$ of a NOR gate with inputs $A$ and $B$ is given by:
  $$ Q = \overline{A + B} $$
  - This means the output is the negation of the OR operation on inputs $A$ and $B$.

Understanding these symbols and their functions is crucial for identifying and working with logic gates in electronic circuits.