[单项选择]Think of the ocean on a calm day. Ignoring the rise and fall of the waves, you might imagine the surface was dead flat the whole way across. You’d be wrong. Hills and valleys are as much as a feature of the sea as the land, although on a much smaller scale.
These undulations have a variety of causes. Tides, currents, eddies, winds, river flow and changes in salinity and temperature push the sea level up in some places and down in others by as much as 2 meters. Ever tried swimming uphill
How do we map these oceanic hills and valleys First, we need to know what the planet would look like without them. This is where the geoid (大地水准面) comes in. It is a surface where the Earth’s gravitational potential is equal and which best fits the global mean sea level. It is approximately an ellipsoid, though uneven distribution of mass within the Earth means that it can vary from this ideal by up to 150 meters.
The geoid represents the shape the sea surface would be if the oceans were n
A. the Earth’s gravitational potential
B. the uneven distribution of mass on the Earth
C. the global mean sea level
D. the surface of the Earth

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