Which is an external stimulus? a.)itch b.)rain c.)thirst d.)hunger

rain.

because that's out of surface, itch / thirst / hunger are in surface becasue they are in our bodies.

hope this helps.

rain.

because that's out of surface, itch / thirst / hunger are in surface becasue they are in our bodies.  

hope this helps

allowed for the prediction of the properties of missing elements.