Fog is formed by

Options

A. Smoke
B. Condensation of water vapours
C. Condensation of water vapours over dust particles
D. Both A and C

Answer

### Step-by-Step Solution:

1. **Understanding Fog**: Fog is a weather phenomenon that occurs when water vapor in the air condenses into tiny liquid water droplets, creating a cloud that is close to the ground.

2. **Conditions for Fog Formation**: Fog typically forms in cooler temperatures, especially during the winter months. This is because colder air can hold less moisture than warmer air, leading to condensation.

3. **Process of Condensation**: When the temperature drops, the water vapor in the air cools down. As it cools, it reaches a point where it can no longer remain in its gaseous state and begins to condense into tiny liquid droplets.

4. **Formation of Fog**: These tiny liquid droplets cluster together, and when they are numerous enough, they become visible as fog. This is why fog is more common in winter when temperatures are lower.

5. **Distinction from Smog**: It’s important to note that fog is different from smog. Smog is a combination of smoke and fog, which contains pollutants and is often associated with urban areas. Fog does not necessarily contain these pollutants.

6. **Conclusion**: Therefore, the correct answer to the question "Fog is formed by" is the condensation of water vapor.