The substance which allows heat radiations to pass through it is

Options

A. Iron
B. Water vapour
C. Wood
D. Dry air

Explanation

To solve the question regarding the substance that allows heat radiations to pass through it, we will follow a systematic approach.

### Step-by-Step Solution:

1. **Understanding Heat Radiation**: Heat radiation refers to the transfer of thermal energy in the form of electromagnetic waves, primarily infrared radiation. This process does not require a medium, but certain materials can either allow or block this radiation.

2. **Defining Terms**:
- **Diathermonous Substances**: These are materials that allow heat radiation to pass through them. Examples include certain gases and transparent materials.
- **Athermonous Substances**: These do not allow heat radiation to pass. Examples include metals and opaque materials.

3. **Identifying Examples**:
- **Diathermonous Examples**: Quartz and dry air are good examples of diathermonous substances. They allow heat radiation to pass through.
- **Athermonous Examples**: Metals like iron and materials like wood do not allow heat radiation to pass through.

4. **Conclusion**: Based on the definitions and examples, the substance that allows heat radiation to pass through it is **dry air**.

### Final Answer:
The substance which allows heat radiations to pass through it is **dry air**.

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