What is pollution?

Options

A. Presence of harmful substances in environment
B. Increase in the number of people on Earth
C. Occurrence of natural disasters
D. Decrease in wildlife

Explanation

### Step-by-Step Solution:

1. **Definition of Pollution**: Pollution is defined as any undesirable change in the environment. This change can be in the physical, chemical, or biological properties of the environment.

2. **Causes of Pollution**: The undesirable changes in the environment are often caused by the presence of harmful substances, known as pollutants. These pollutants can be natural or man-made and can have detrimental effects on health and the ecosystem.

3. **Types of Pollution**: There are several major types of pollution, including:
- **Air Pollution**: Contamination of the air by harmful substances.
- **Water Pollution**: Contamination of water bodies such as rivers, lakes, and oceans.
- **Soil Pollution**: Degradation of the earth's land surfaces.
- **Noise Pollution**: Excessive noise that may harm human or animal life.
- **Radioactive Pollution**: Contamination from radioactive substances.

4. **Conclusion**: Based on the definition and types of pollution, the correct answer to the question "What is pollution?" is the presence of harmful substances in the environment.