*Question G5A02:
What is reactance?*  

*Correct Answer is  B: Opposition to the flow of alternating current caused by capacitance or inductance*  

### AI Response: (may contain errors)  

Reactance is a key concept in AC (alternating current) circuits, and it refers to the opposition that capacitors and inductors present to the flow of alternating current. 

### Correct Answer Explanation:
- **Correct Answer: B**: Opposition to the flow of alternating current caused by capacitance or inductance.
  - **Why it's correct**: Reactance arises in AC circuits due to the properties of capacitors and inductors. Capacitors store energy in an electric field and oppose changes in voltage, while inductors store energy in a magnetic field and oppose changes in current. This opposition is frequency-dependent, meaning it varies with the frequency of the AC signal.

### Incorrect Answer Explanations:
- **Choice A**: Opposition to the flow of direct current caused by resistance.
  - **Why it's incorrect**: This describes resistance, not reactance. Resistance is the opposition to current flow in both AC and DC circuits, but it does not account for the frequency-dependent behavior of capacitors and inductors.

- **Choice C**: Reinforcement of the flow of direct current caused by resistance.
  - **Why it's incorrect**: This statement is misleading. Resistance does not reinforce current; it opposes it. Reactance specifically pertains to AC circuits, not DC.

- **Choice D**: Reinforcement of the flow of alternating current caused by capacitance or inductance.
  - **Why it's incorrect**: This is also misleading. Reactance does not reinforce current flow; it opposes it. Capacitors and inductors impede the flow of AC, which is the essence of reactance.

### Useful Formulas:
- **Capacitive Reactance ($X_C$)**:
  $$X_C = \frac{1}{2 \pi f C}$$
  where:
  - $f$ = frequency in hertz
  - $C$ = capacitance in farads

- **Inductive Reactance ($X_L$)**:
  $$X_L = 2 \pi f L$$
  where:
  - $L$ = inductance in henries

### Summary:
- Reactance is the opposition to AC caused by capacitors and inductors.
- It is distinct from resistance, which applies to both AC and DC.
- Understanding reactance is crucial for analyzing AC circuits and their behavior.