The argument tells us that one can determine the rattlesnake's age by looking at the number of section in its rattle as a new section is formed every time a rattlesnake molts. This conclusion only requires that the rattlesnake molts at regular intervals, rather than once a year. It is possible that rattlesnake always molts twice a year for instance, so one would just divide the number of sections by two to determine the age. Since a 1 to 1 ratio of years/ sections is not necessary, we can eliminate (A). (E) on the other hand tells us that rattlesnake molts as often when the food is scare as when it is plentiful, telling us that rattlesnakes molt at regular intervals regardless of food conditions. This is a necessary assumption because if we negate it, we can no longer tell that rattlesnakes molt at regular intervals, i.e. perhaps they molt twice as often when food is scarce, and then it would be impossible to determine the rattlesnake's age.