The sea snail used in the experiment did not differ significantly from other members of its species in its reaction to external stimuli. Tensing of the muscular "foot" in sea snails is an instinctual rather than a learned response to ocean turbulence. All sea snails react to ocean turbulence in the same way as the sea snail in the experiment did. The appearance of a bright light alone would ordinarily not result in the sea snail's tensing its "foot. "
In an experiment, biologists repeatedly shone a bright light into a tank containing a sea snail and simultaneously shook the tank. The snail invariably responded by tensing its muscular "foot, " a typical reaction in sea snails to ocean turbulence. After several repetitions of this procedure, the snail tensed its "foot" whenever the biologists shone the light into its tank, even when the tank was not simultaneously shaken. Ttherefore, the snail must have learned to associate the shining of the bright light with the shaking of the tank.
Which one of the following is an assumption required by the argument?