RC help needed

[g2m0a0t9]

The Pan-American land bridge, or
isthmus, connecting North and South
America was formed volcanically long
after dinosaurs became extinct. The
isthmus cleaved populations of marine
organisms, creating sister species. These
twin species, called "geminates," then
evolved independently. Scientists observe,
for example, that Pacific pistol shrimp no
longer mate with those from the Atlantic
Ocean. Yet the two oceans had already
begun to form their distinctive personali-
ties long before the isthmus was fully
formed. As the seabed rose, Pacific
waters grew cooler, their upswelling
currents carrying rich nutrients, while the
Atlantic side grew shallower, warmer,
and nutrient poor. In fact, it was these
new conditions, and not so much the
fully-formed isthmus, that spawned
changes in the shrimp population.
For terrestrial life, the impact of the
isthmus was more immediate. Animals
traversed the newly formed bridge in
both directions, although North Ameri-
can creatures proved better colonizers--
more than half of South America's
mammals trace direct lineage to this
so-called Great American Biotic Ex-
change. Only three animals--the armadillo,
opossum, and hedgehog--
survive as transplants in the north today.

Which of the following statements is most readily inferable from the information in the passage?


a. Species of marine organisms in the Atlantic Ocean number fewer today than before the formation of the Pan-American isthmus.

b. The number of terrestrial animal species in South America today exceeds the number prior to the formation of the Pan-American isthmus.

c. Of the indigenous South American species that migrated north across the Pan-American isthmus, more than three survive to this day.

d. Since the formation of the Pan-American isthmus, fewer terrestrial animals have traveled north across the isthmus than south.

e. As the Pan-American isthmus began to form, most pistol shrimp migrated west to what is now the Pacific Ocean.

Please indicate the correct answer

Thanks!

[madboy]

IMO B

Reasons :

A - The passage says after the formation of the isthmus, as the sea bed rose, the Atlantic side grew shallower, warmer and poor in nutrients - however from this it cannot be inferred that this phenomenon led to a reduction in number of species of marine organisms. The passage ony suggests that there was a change in the population of marine organisms.

B - Terrestrial organisms migrated in both directions across the the isthmus. However the northern species proved better colonizers, which means a majority of them survived after the migration. So definitely the number of species today in South America exceeds the number before the formation of the isthmus. So B is correct

C - This is in direct contradiction with the last sentence of the passage.

D - The passage does not suggest in which direction the influx of migrating animals was more. It only says that while majority of the species who migrated from north to south survives till date, only three among those who migrated from south to north have managed to survive till date.

E - Absolutely no information in the passage suggesting this.

[Fiver]

The last statement implies that of all species that travelled from south to north only 3 survive in the north today.
Though this makes D somewhat good. D goes too far to say that fewer animals have travelled north.
The best one is indeed B. Clue is the statement :

'more than half of South America's mammals trace direct lineage to this so-called Great American Biotic Ex-change', suggesting that the no. of species has grown by more than atleast half as the earlier no.