LSAT-16-2-24

snow_mountain

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 must 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.
24. 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


答案C。用了排除法，但为什么是C实际也不是太明白。没看懂里面的 bridge.

番茄炒蛋

lava flows differing by thousands of years in age often have very similar directions of
magnetization,
这句话很重要，因为它再过了几千年以后磁场的方向又回去了，如果要说明这个过程很慢长，那么前提就是这个变化要很有规律，只有按照这个规律变它个5000年，然后变成了一个方向，才能得出这其中的确是有一个很漫长的变化。
如果今天朝左，明天又朝右，这样的变化根本就没有规律可循啊，更谈不上take place very
gradually over hundreds of thousands of years