Passage Summary: Scientists repeatedly shook a snail's tank while shining a light on it until the snail started bracing itself just from the light, leading them to believe the snail connected the two events.

Conclusion: The snail's reaction to the light alone proves it has linked the light to the physical shaking of the tank.

Reasoning: Initially, the snail only tensed up when the tank was shaken, but after repeated pairing with light, it tensed up when only the light was present.

Analysis: The argument assumes that the light itself isn't the direct cause of the foot-tensing. We need to bridge the gap between the snail reacting to light and the snail reacting because it expected a shake. Look for an answer choice that rules out the possibility that the snail simply became sensitive to light over time regardless of the shaking.