Necessary Assumption Question: The Rattlesnake Folktale

LSAT Logical Reasoning Question Types: A New Approach started off this series. This is the second part, where I go into a particular example.

Let's look at one of my favorite Logical Reasoning questions: the Rattlesnake Folktale question. It's PrepTest 30 - December 1999 LSAT, Section 2, Question 22 - page 60 in Next 10.

We know this is a necessary assumption question because it says "which