Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

There are two basic types of glaciers, those that flow outward in all directions with little regard for any underlying terrain and those that are confined by terrain to a particular path.

The first category of glaciers includes those massive blankets that cover whole continents, appropriately called ice sheets. There must be over 50,000 square kilometers of land covered with ice for the glacier to qualify as an ice sheet, when portions of an ice sheet spread out over the ocean, they form ice shelves.

About 20,000 years ago the Cordilleran Ice sheet covered nearly all the mountains in southern Alaska, western Canada, and the western United States. It was about 3 kilometers deep at its thickest point in northern Alberta. Now there are only two sheets left on Earth, those covering Greenland and Antarctica.

Any domelike body of ice that also flows out in all directions but covers less than 50,000 square kilometers is called an ice cap. Although ice caps are rare nowadays, there are a number in northeastern Canada, on Baffin Island, and on the Queen Elizabeth Islands.

The second category of glaciers includes those of a variety of shapes and sizes generally called mountain or alpine glaciers. Mountain glaciers are typically identified by the landform that controls their flow. One form of mountain glacier that resembles an ice cap in that it flows outward in several directions is called an ice field. The difference between an ice field and an ice cap is subtle. Essentially, the flow of an ice field is somewhat controlled by surrounding terrain and thus does not have the domelike shape of a cap. There are several ice fields in the Wrangell. St. Elias, and Chugach mountains of Alaska and northern British Columbia.

Less spectacular than large ice fields are the most common types of mountain glaciers: the cirque and valley glaciers. Cirque glaciers are found in depressions in the surface of the land and have a characteristic circular shape. The ice of valley glaciers, bound by terrain, flows down valleys, curves around their corners, and falls over cliffs.

What does the passage mainly discuss?