When volcanic lava solidifies, it becomes uniformly magnetized in the

The main conclusion of the Argument - The Earth's magnetic field has changed over time.

The Author relies on 2 facts to make this Suggestion.

Line 1 - General fact = Solidified Lava gets magnetized to the direction of the Magnetic Field.

Line 2 - Another Fact = Solidified Lava from different time periods point to different directions.

To make the claim that the Earth's Magnetic Field has changed, the Author has considered that the change in directions must be due to the Earth's Magnetic Field only. In this case, it happens to be a change in direction.

C...strengthen the statement

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The argument is based on the observation that solidified lava flows from different volcanoes, which erupted at different times over millions of years, have significant differences in the direction of magnetization. This observation leads to the conclusion that the direction of the Earth's magnetic field has changed over time and that this change must occur gradually over hundreds of thousands of years.

The assumption underlying this argument is that the magnetization of lava flows accurately reflects the direction of the Earth's magnetic field at the time of their solidification. If this assumption were not true, then the argument's conclusion about changes in the Earth's magnetic field based on differences in lava magnetization would be called into question.

Among the given options:

(A) This option introduces the idea of using lava to measure the Earth's magnetic field, but it doesn't address the assumption that the magnetization of lava accurately reflects the Earth's magnetic field. It's not directly relevant to the argument.

(B) This option discusses the consistency of lava produced by a single volcano, which is not relevant to the assumption regarding whether lava magnetization accurately reflects the Earth's magnetic field.

(C) This option is directly relevant to the assumption because it suggests that not all solidified lava changes its magnetization unpredictably, which is necessary for the argument's conclusion to hold. If all lava changed its magnetization unpredictably, then the argument's basis for inferring changes in the Earth's magnetic field would be undermined.

(D) This option discusses the frequency of volcanic eruptions, which is not directly related to the assumption about the accuracy of lava magnetization.

(E) This option introduces the idea of lava flows picking up magnetized rocks, but it doesn't address the assumption about the accuracy of lava magnetization in reflecting the Earth's magnetic field.

The correct answer is (C) because it addresses the critical assumption that not all solidified lava changes the direction of its magnetization unpredictably, which is necessary for the argument's conclusion to hold.