An atmosphere

Options

A. is a unit of pressure
B. is a unit of force
C. gives us an idea of composition of air
D. is the height above which there is no air

Answer

### Step-by-Step Solution:

1. **Understanding Atmospheric Pressure**:
- Atmospheric pressure is the force exerted by the weight of air above a surface. It is the pressure exerted by the atmosphere on any object within it.

**Hint**: Remember that atmospheric pressure is related to the weight of the air above us.

2. **Identifying the Nature of the Question**:
- The question asks for the correct definition or characteristic of an atmosphere from the given options.

**Hint**: Look for the option that directly relates to pressure rather than force or composition.

3. **Analyzing the Options**:
- **Option 1**: Is a unit of pressure - This is correct because atmospheric pressure is measured in units such as Pascals (Pa), and 1 atmosphere (atm) is a standard unit of pressure.
- **Option 2**: Is a unit of force - This is incorrect because atmospheric pressure is not a force; it is the pressure resulting from a force applied over an area.
- **Option 3**: Gives us an idea of the composition of air - This is incorrect as atmospheric pressure does not provide specific information about the composition of air.
- **Option 4**: Is the height above which there is no air - This is also incorrect because the atmosphere extends to a significant height above the Earth, but it does not define a specific height.

**Hint**: Evaluate each option critically to determine if it accurately describes atmospheric pressure.

4. **Conclusion**:
- The only correct option is that an atmosphere is a unit of pressure.

**Hint**: Confirm your choice by recalling the definition of atmospheric pressure and its units.

### Final Answer:
An atmosphere is a unit of pressure.