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

Choices:  
A: 1  
B: 2  
C: 3  
D: 4    

Correct Answer is  D: 4  

To identify a NOR gate symbol:
- A NOR gate is an OR gate with an inversion bubble on the output.
- In Boolean form: $$Y = \overline{A + B}$$

Why D: 4 is correct:
- Symbol 4 shows the OR-gate shape (curved input side, pointed output side) with a small circle (bubble) on the output. The bubble means inversion (NOT), so it’s an OR followed by NOT → NOR.

Why the others are incorrect:
- A: 1 — AND gate (D-shaped, flat input side) without an output bubble → AND, not NOR.
- B: 2 — OR gate without an output bubble → plain OR, not NOR.
- C: 3 — AND gate with an output bubble → NAND, not NOR.

Useful rules of thumb:
- Bubble = inversion (NOT).
- OR shape + bubble on output → NOR: $$\overline{A + B}$$
- AND shape + bubble on output → NAND: $$\overline{A \cdot B}$$
- Extra curved line on the input side of an OR shape indicates XOR; add a bubble to make XNOR.