Which of the following statement is not a statement in logic?

Options

A. Earth is a planet
B. Plants are living object.
C. `sqrt(-9)` is a rational number.
D. I am lying

Answer

To determine which of the given statements is not a statement in logic, we need to analyze each option to see if it can be classified as either true or false.

### Step-by-Step Solution:

1. **Understand what constitutes a logical statement**: A statement in logic must be a declarative sentence that can be clearly classified as either true or false.

2. **Evaluate Option A**:
- Statement: "Earth is a planet."
- Analysis: This statement is true. It can be classified as a logical statement.
- Conclusion: This is a logical statement.

3. **Evaluate Option B**:
- Statement: "Plants are living objects."
- Analysis: This statement is also true. It can be classified as a logical statement.
- Conclusion: This is a logical statement.

4. **Evaluate Option C**:
- Statement: "Square root of minus 9 is a rational number."
- Analysis: This statement is false because the square root of a negative number is not a rational number. It can still be classified as a logical statement.
- Conclusion: This is a logical statement.

5. **Evaluate Option D**:
- Statement: "I am lying."
- Analysis: This statement creates a paradox. If the speaker is lying, then the statement is true, which means they are not lying. Conversely, if they are telling the truth, then they are lying. This statement cannot be classified as simply true or false.
- Conclusion: This is not a logical statement.

### Final Answer:
The statement that is not a statement in logic is **Option D: "I am lying."**

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