Answer to Question 1
The two common elevations represent the two types of crust on Earth. Continental crust underlies most of the area at 0.5 km above sea level and oceanic crust underlies the area at 4.5 km below sea level. Continental crust is thicker and less dense than oceanic crust and so it sits up higher in the mantle, averaging 0.5 km above. Oceanic crust is more dense and thinner than continental crust, so it sits lower in the mantle, averaging 4.5 km below sea level.
Answer to Question 2
Both models require that the crust subside due to the weight of the glacial ice. The Pratt-Airy model requires that only the crust beneath the glaciers subsides and the crust adjusts rather quickly as the ice melts and would be completely adjusted after all the ice is gone. The flexural model requires that an area larger than the area covered with glacial ice will subside and that the adjustment to the melting of the ice will be slow. Data supports the flexural isostasy model because areas larger than those covered by ice are still undergoing isostatic adjustment today.
Mount Logan, Yukon. Environmental conditions and organisms vary greatly from low to high elevations
Nuclear division at the DNA level
Zhang Huan, To Raise the Water Level in a Fishpond.
Reaction 7: The First Substrate-Level Phosphorylation
World Mean Sea-Level Temperatures in January
The quaternary level of protein structure as seen in hemoglobin