A more comprehensive definition of "sea level," one useful to cartographers, requires a broad understanding of various temporal and physical factors. First, consider the history of sea level: the position at which sea and land meet, averaged over tidal stages and coastlines, has been remarkably stable over time. Current sea level measures, though having risen very slightly over the last century, are approximately the same as they were 35,000 years ago. Thus, the referent for terrestrial altitudes and oceanic depths has been largely consistent for millennia, though cartographers' ability to make accurate measurements has, of course, improved with advances in technology. Through the processes of imaginative visualization and computer generation, much knowledge has been gained regarding the predictable nature of sea level.

The oceans are governed primarily by Earth's gravity, which is strongest at the poles because of Earth's slightly ellipsoid shape, the result of billions of years of rotation. What keeps the oceans from pooling at the poles, drawn there by the greater gravitational forces, is the centrifugal force generated by Earth's rotation. This force is strong enough to cause a bulge of ocean about eight kilometers high around Earth's equator. Using sophisticated computer models, today's cartographers can postulate how the intersection of sea and land would change should the Earth ever cease to rotate on its axis. Over time, the bulge of ocean at the equator, no longer held in place by the centrifugal forces, would flatten out. Drawn by the pull of the two poles, Earth's main bodies of water would eventually become circumpolar, gathered in two massive pools around the top and bottom, so to speak, of the globe, leading to the emergence of a supercontinent around the planet's equator.

1.According to the passage, which is true of the physical forces governing sea level?
A.Gravity, along with rotation, is responsible for Earth's slightly ellipsoid shape.
B.Centrifugal deterioration will eventually give rise to an enormous supercontinent.
C.The centrifugal force is the dominant influence on the shape of Earth's oceans.
D.The equatorial bulge is primarily a result of gravitational pooling at Earth's poles.
E.Gravitational forces have more influence on the oceans than does centrifugal force.


2.The author makes all of the following statements about historic sea levels EXCEPT
A.Variations in them have been noted in the last 35,000 years.
B.Their consistency has allowed cartographers to improve their measurements.
C.While mutable, they have, broadly speaking, maintained stability for millennia.
D.They are measured at many times and places to obtain a useful average.
E.Measurements of depth and elevation are made relative to their height.


3.The passage is chiefly concerned with
A.imparting an understanding of the effect of Earth's physical forces on sea life
B.arguing that the long-term stability of sea level assists in cartographic accuracy
C.attempting to compose an explanation of one of Earth's topographical variables
D.integrating hard data and thought experimentation to better visualize sea level
E.positing a worst-case scenario to educate readers about Earth's fragile balance