LSAT 22-1-20 原文看不出答案这个含义。

siebel

20. Until about 400 million years ago, fishes-the first true swimmers-were jawless. Their feeding methods were limited to either sucking in surface plankton or sucking in food particles from bottom mud. With the development of biting jaws, however, the life of fishes changed dramatically, since jaws allowed them actively to pursue prey, to seize it in their jaws, and to manipulate it between their teeth. The jawed fishes then developed along two main lines one retained cartilage for its skeletons, for example, sharks and rays: the other adopted bone as its principal skeletal material. From the latter group evolved the most abundant and diverse of all of today's vertebrate groups. the "teleosts" some 21,000 species, which vary from barracudas to sea horsesl

If all of the statements in the passage are true which one of the following must also the true?

(A) Fish are the primary prey of all jawed fishes
(B) The jawless fishes did not prey upon other fish
(C) Teleosts do not feed upon particles found in bottom mud
(D) Jawless fishes did not have cartilage as their skeletal material
(E) Jawless fishes became extinct approximately 400 million years ago

原文可以读出：
1.400M年前，jawless不吃别的鱼；
2.400m年后，jaw鱼可以prey。

答案B的意思我理解是：jawless鱼不吃别的鱼。400前对，400后原文并没有说jawless鱼灭绝了或即便活着也不吃别的鱼。故答案b困惑。

albert

我用排除法做的：
A primary prey 原文没提到
C 原文只是说他们可以抓鱼了，但没有说他们不吃泥巴里面的东东（象吃怀旧农家菜一样）
D 从原文“one retained cartilage for its skeletons”可以排除
E 原文没有说他们灭绝了。

我们在看B吧：
400年前， Their feeding methods were limited to either sucking in surface plankton or sucking in food particles from bottom mud.
之后，JAW横空出世，好处多多，改变了他们的生活。since jaws allowed them actively to pursue prey(说明JAW是它们能吃鱼的必要条件)。。那么没了JAW，自然没法吃鱼。

不过，如果since jaws allowed them actively to pursue prey说明JAW是它们能吃鱼的充分条件条件，则B不一定成立了。。也许它又进化出另一种更先进的抓鱼工具，比如红外线抓鱼。

不过，综合考虑，我还是觉得B最好。因为后一种可能性很小了。