【单选题】

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 theEarth’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 theEarth 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 net moving and affected only by gravity. Thus it can be used as a reference to measure any deviations in the ocean surface height that aren’t caused by gravity—the hills and valleys, for instance, or any regional increase in sea level.
So how do you measure the geoid and the ocean’s irregular topography It’s complicateD、Geophysicists calculate the geoid using data on variation in gravitational acceleration from several dozen satellites.
The hills and valleys of the oceans are all very interesting, but can the geoid tell us anything more significant about the state of the planet It certainly can. Knowing accurately where the geoid lies and how the Ocean surface deviates from it will help meteorologists spot changes in Ocean currents associated with climate change. The circumpolar current aroundAntarctic is one they are particularly interested in.
It can also predict local climate variations produced by events such asEl Nino,El Nino keeps warm water that would normally move westwards close to the coast of SouthAmerica, deprives SoutheastAsia of its monsoon rains, and increases rainfall on the west coast of theAmetlcA、Since temperature changes cause changes in sea level, geoid-watchers should be able to prepare us before it strikes.
What can we learn aboutEl Nino from the last paragraph of the passageA．It keeps warm water move westwards.
B．It brings drought to SoutheastAsiA、
C．It brings more rain to theAtlantic Ocean.
D．It helps sea level to remain steady.