Which of the following is not a combustible substance?

Options

A. camphor
B. glass
C. straw
D. alcohol

Answer

To determine which of the following substances is not a combustible substance, we need to understand what a combustible substance is. A combustible substance is one that can burn in the presence of air (specifically oxygen).

Let's analyze the options provided:

1. **Camphor**: This is a substance that can burn. It can also undergo sublimation, where it changes from solid to vapor without becoming liquid, but it is still considered combustible because it can ignite and burn in the presence of oxygen.

2. **Glass**: Glass is an inorganic material that does not burn. It does not react with air or oxygen, which means it cannot undergo combustion. Therefore, glass is not a combustible substance.

3. **Straw**: Straw is made from grass, which is organic and can burn easily in the presence of oxygen. Thus, straw is a combustible substance.

4. **Alcohol**: Alcohol has a low boiling point and is flammable. It burns readily in the presence of oxygen, making it a combustible substance.

After analyzing all the options, we conclude that the substance that is not combustible is **glass**.

### Final Answer: Glass

---