The astronomical event that represents the great serpent mound is Halley's Comet.
I believe the answer is D. The NASA mission which sent a probe to collide with a comet's nucleus is the Deep Impact. It is the first mission sent in order to see the interior of a comet. The comet that was aimed was called the Comet Tempel 1.
Incoming solar radiation is almost double at the equator then at the poles, causing the water closer to the equator to be warmer. The heat difference also causes the oceans to swirl so the water that is around the equator will eventually end up at the poles and keep circulating around.
Uneven warming by the sun I believe
