Air is regarded as a mixture because

Options

A. its pressure may vary
B. its temperature may change
C. its volume changes under different conditions
D. its composition may vary

Explanation

### Step-by-Step Solution:

1. **Definition of Air**: Air is the invisible mixture of gases that surrounds the Earth. It is primarily composed of nitrogen (about 78%), oxygen (about 21%), and small amounts of other gases such as argon, carbon dioxide, and water vapor.

2. **Physical Separation**: The components of air are not chemically bonded together; they are physically mixed. This means that each gas retains its own properties and can be separated from the mixture through physical processes.

3. **Variable Composition**: The composition of air can change depending on various factors such as location, altitude, and weather conditions. For example, in a humid area, the amount of water vapor in the air can increase, while in a dry area, it can decrease.

4. **Presence of Other Gases**: Besides nitrogen and oxygen, air contains trace amounts of other gases, including noble gases and pollutants. This variety of components further supports the classification of air as a mixture.

5. **Conclusion**: Because air is a combination of different gases that are not chemically bonded and can vary in composition, it is classified as a mixture.