Conclusion: it must be that the direction of the Earth's magnetic field has changed over time.
Why? We know that when lava solidifies, it becomes uniformly magnetized in the direction in which the Earth's magnetic field points. We are told that lava flows differing by thousands of years in age often have very similar directions of magnetization.
We want to weaken the conclusion of the argument.
If we take answer choice (D) to be true, then we know that in the span of the two weeks it took for this one lava flow to solidify, the surface of the flow has a different direction of magnetization than its center.
So as we can see, in the mere span of two weeks, the Earth's magnetization changed significantly, as seen by the very different directions in the same lava flow.
Hope that helps! Please let us know if you have any other questions.
