Discussion

Explanation:

In the entire passage, the author is giving various possible explanations to prove why the white-lipped variety of grove snails is found only in Ireland and the Pyrenees.
Option 1 is wrong as the second sentence of the second paragraph clearly eliminates the possibility of the white-lipped grove snails being wiped out in other places except Ireland and the Pyrenees.
Option 2 is incorrect as the clearly states that evolution is not responsible for the white-lipped grove snails in both the places.
Option 3 is the main aim of the author in writing the passage. Retain it.
The last sentence of the second paragraph clearly states that if the snails had naturally colonized Ireland, they would have similar genetic traits across other snails in Europe. But since this is not the case, migration of snails can be negated as an explanation. Thus option 4 can be negated.
Hence, the correct answer is option 3.

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