*Question E7A01:
Which circuit is bistable?*  

*Correct Answer is  C: A flip-flop*  

### AI Response: (may contain errors)  

A bistable circuit is one that has two stable states and can remain in either state indefinitely until an external input causes it to switch states. Let's examine the choices:

### Correct Answer: C: A flip-flop
1. **Why the correct answer is correct:**
   - A flip-flop is a type of bistable multivibrator. It has two stable states and can store one bit of information. It remains in one state until it receives a trigger to switch to the other state. This makes it ideal for memory storage and digital logic applications.

### Incorrect Choices:
2. **Why the other choices are incorrect:**
   - **A: An AND gate**
     - An AND gate is a combinational logic circuit, not a bistable circuit. It outputs a high signal only when all its inputs are high. It does not have stable states; its output depends solely on the current inputs.
   - **B: An OR gate**
     - Similar to the AND gate, an OR gate is also a combinational logic circuit. It outputs a high signal if at least one of its inputs is high. It does not have stable states and its output is determined by the current inputs.
   - **D: A bipolar amplifier**
     - A bipolar amplifier is an analog device used to amplify signals. It does not have stable states like a bistable circuit. Its output is a continuous function of its input, not a discrete state.

### Summary:
- **Bistable Circuit:** A circuit with two stable states, such as a flip-flop.
- **Combinational Logic Circuits (AND, OR):** Output depends on current inputs, not stable states.
- **Analog Devices (Amplifiers):** Output is a continuous function of input, not discrete states.