A: The earth's magnetic field not only varies from point to point in space, it also changes with time
R: The earth's core is known to contain iron yet geologists do not regard this as a source of the earth's magnetism.

Options

A. If both Assertion & Reason are the true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reson is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to evaluate the assertion (A) and the reason (R) provided in the statement. Let's break it down step by step.

### Step 1: Analyze the Assertion (A)
The assertion states that "The earth's magnetic field not only varies from point to point in space, it also changes with time."

- **Point 1**: The Earth's magnetic field does indeed vary from point to point in space. This is a well-established fact in physics, as the magnetic field strength and direction can differ based on geographic location.

- **Point 2**: The Earth's magnetic field also changes with time. This is true as well; there have been documented changes in the Earth's magnetic field over geological time scales, including pole reversals.

**Conclusion for Step 1**: The assertion is true.

### Step 2: Analyze the Reason (R)
The reason states that "The earth's core is known to contain iron yet geologists do not regard this as a source of the earth's magnetism."

- **Point 1**: It is correct that the Earth's core contains iron, primarily in a molten state.

- **Point 2**: Geologists do not consider this molten iron as a source of the Earth's magnetism because molten iron is not ferromagnetic. For a material to contribute to magnetism, it typically needs to be in a solid state and exhibit ferromagnetic properties.

**Conclusion for Step 2**: The reason is true.

### Step 3: Determine the Relationship Between A and R
Now we need to determine if the reason is the correct explanation for the assertion.

- The assertion discusses the variation of the Earth's magnetic field both spatially and temporally.
- The reason explains why the molten iron in the Earth's core is not considered a source of magnetism, but it does not directly explain why the magnetic field varies in space and time.

**Conclusion for Step 3**: The reason is true, but it does not correctly explain the assertion.

### Final Conclusion
Both the assertion and the reason are true, but the reason is not the correct explanation of the assertion. Therefore, the correct option is:

**B**: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

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