Which of the following change cannot be reversed?

Options

A. Hardening of cement
B. Freezing of ice cream
C. Opening a door
D. Melting of chocolate

Explanation

### Step-by-Step Solution:

1. **Understanding the Types of Changes**:
- There are two main types of changes: reversible and irreversible.
- Reversible changes allow us to return to the original state after the change, while irreversible changes do not.

2. **Identifying Irreversible Changes**:
- An example of an irreversible change is the hardening of cement.
- When cement is mixed with water, it hardens and cannot revert back to its original powdered form.

3. **Analyzing Other Changes**:
- Freezing of ice cream is reversible because if it melts, it can be refrozen.
- Opening and closing of a door is reversible since you can return it to its original position.
- Melting of chocolate is also reversible; it can solidify again when cooled.

4. **Conclusion**:
- Among the changes mentioned, the hardening of cement is the only irreversible change.

### Final Answer:
The change that cannot be reversed is the **hardening of cement**.

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