我不得不出来说两句了.这道题的答案是E. 这道题的逻辑是, 题目假设最初的时候 salt level is 0. S(0)=0, now, S(t)=C (some constant) the author says we can decide the max(t) = (C-0) / v where v is the somehow determined.(but this is not important here. you will see in just a sec.)
now, remember, this max(t) is actually the time the ocean take to accumulate to current salt level C from 0. It is not "the maximum age of the Earth’s ocean" by default. Not unless salt level of the ocean never go back to 0 again. so here is a case, S(0)=0, S(100)=C and some how salt level in ocean is used up and S(200)= 0. then salt starts to cumulate again and now we go back to S(300)=C. now, use (C-0)/v you will get 100 yrs. But in fact, the age of ocean is 300.
now look at answer E, "None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans." these bold faces carry the important assumption you need to estimate the age of ocean.
2。原文计算海洋最大年数的方法：current salt levels （即现在海洋的盐的浓度或水平）除以resulting increase 。因为起点是salt-free state，即零。所以，该题可攻击的因素只剩下current salt levels （即现在海洋的盐的浓度或水平），即current salt levels 是否能反映真实的现在应该有的浓度，即在该速率下，花多少年所能达到的浓度。如果如E取非所说，即有些河流流入的盐没了，则目前的salt levels 低估了，算出的年度偏小，结论不能成立。
A中只说明了某些年分进入大海的盐量会有波动，但是总量是基于初始值为0的条件下的，也就是说个别年份盐量的波动不会影响到总量。the resulting increase in salt levels in the oceans over the past hundred years，暗示增加盐量是已知的
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发表于 2006-12-31 09:00:00
