Final Quest

"We met Queen Jade," you say. And you tell the king what she told you.
"We need the stones in your keeping," the green dragon adds.
The king looks at you both. "You must agree to complete tasks to win the stones," he tells you.
"We agree," the green dragon answers.
The king waves his insubsantial hand towards five identical goblets. "Four of these goblets have poison in them," he says to the green dragon. "Only one contains harmless water. Choose the one with water in, and live."
The green dragon looks down at you. "Which do you think I should choose?" he asks.