A scientific study provides evidence that crows are capable of recognizing threatening people and can even pass their...

Emil-Kunkin on August 1 at 07:07PM

The author is trying to show that crows are able to communicate information about dangers to other crows. To support this the author cites an instance in which crows were presented with a danger and then years later, attacked that same danger when it reappeared.

In order for the argument to work we need to make sure the crows actually did communicate the danger. If the crows who attacked were the same crows who were trapped, then no communication need to have occurred. The only way we can prove there was communication is to show that those two groups of crows are mutually exclusive. This is what A does.