What is the feature known as the "Great Dark Spot" ofNeptune?

		It is an apparently permanent feature about five times the sizeof Earth, similar to the Great Red Spot of Jupiter, near Neptune'ssouth pole.
		It was a dark hole in the upper atmosphere left by the collisionof the comet Shoemaker-Levy 9.
		It was an apparently temporary feature about the size of Earth,similar to the Great Red Spot of Jupiter, but disappeared within afew years.
		It is a dark surface feature on the surface snow layers causedby radiation discoloration of the older layers.
		It is a permanent discoloration of the north polar region ofNeptune caused by locally prevailing lower surface temperaturesthere.