If we do not clean our teeth and mouth after eating food, then many harmful micro-organisms called A begin to grow and live on the tooth. These micro-organisms act on the substance B present in the left-over food particles sticking to the tooth to form C. The C eats up the parts D and E of the tooth gradually and ultimately make a hole in the tooth. When this hole reaches part F of the tooth, our tooth becomes painful and we get toothache.
What is C?

Answer

To solve the question, we need to identify what "C" is in the context of tooth decay caused by microorganisms acting on leftover food particles. Here's a step-by-step breakdown:

### Step-by-Step Solution:

1. **Understanding the Problem**: The question describes a scenario where leftover food particles stick to the teeth, leading to the growth of harmful microorganisms. These microorganisms act on certain substances in the food.

2. **Identifying Substance B**: The leftover food particles contain various substances, but sugar is a significant component that is often present in food. Therefore, we can conclude that substance B is sugar.

3. **Formation of C**: When microorganisms act on sugar (substance B), they metabolize it and produce acids as a byproduct. These acids are what we refer to as "C."

4. **Effects of C**: The acids produced by the microorganisms eat away at the tooth structure, leading to decay. This decay can eventually create a hole in the tooth.

5. **Conclusion**: Therefore, "C" is the acids produced by the microorganisms when they act on sugar.

### Final Answer:
C is **acids**.