请大家指教！

jiang20w

我有一道lr的假设题不太明白:When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth's magnetic field points. There are significant differences in the direction of magnetization among solidified lava flows from different volcanoes that erupted at different times over the past several million years. Therefore, it muse be that the direction of the Earth's magnetic field has changed over time. Since lava flows differing by thousands of years in age often have very similar directions of magnetization, the change in the direction of the Earth's magnetic field must take place very gradually over hundreds of thousands of years.
The argument that the direction of the Earth's magnetic field has changed over time requires the assumption that
A, only lava can be used to measure the direction of the Earth's magnetic field as it existed in the distant past
B. a single volcano can produce lava of differing consistencies during different eruptions
C. not all solidified lava has changed the direction of its magnetization unpredictably
D. there are fewer volcanic eruptions now than there were millions of years ago
E. as lava flows down the side of a volcano, it picks up magnetized rocks

jiang20w

对的，c是正确答案，谢谢你！

simonshen

这题貌似应该是C吧？
assumption题可以试下“否定法”。
如果把某个assumption否定后，题目结论不成立了，那这个就是题目必须要依赖的assumption。
C否定了就是“all solidified lava has changed the direction of its magnetization unpredictably”。
那如果所有固化的lava的磁性方向都以不可预测的方式变过了，那就不能用这些磁性方向来判断地球的磁场方向了。
其他的选项也可以试下这个否定法，但会发现其他选项如果被否定的话题目的结论是不受影响的。