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 D?

Answer

To solve the question, we need to identify what part D refers to in the context of tooth decay. Let's break down the information provided step by step.

### Step-by-Step Solution:

1. **Understanding the Problem**: The question describes a process that occurs when we do not clean our teeth after eating. Harmful microorganisms (A) grow on our teeth and act on leftover food particles (substance B) to produce an acid (C).

2. **Identifying Substance B**: The leftover food particles that stick to our teeth often contain sugars. These sugars are what the bacteria feed on.

3. **Role of Microorganisms**: The harmful microorganisms (A) consume the sugars (B) and produce acid (C) as a byproduct. This acid is responsible for damaging our teeth.

4. **Effect of Acid on Teeth**: The acid produced by the bacteria begins to erode the outer layer of the tooth, which is known as enamel.

5. **Identifying Part D**: The question asks for part D, which is the part of the tooth that the acid (C) acts upon. Since the acid first attacks the enamel, we can conclude that part D is the enamel.

6. **Consequences of Tooth Decay**: As the enamel is eroded, the bacteria can reach deeper layers of the tooth, such as the dentine and eventually the pulp cavity, leading to pain and toothache when the decay reaches the pulp.

### Final Answer:
**D is enamel.**

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