Which of the following does the passage mainly discuss? What is the purpose of the material in paragraph three (lines 16-19)

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问题 Which of the following does the passage mainly discuss?
What is the purpose of the material in paragraph three (lines 16-19)

选项 A、To define two types of glaciers
B、To contrast glacier ice with non-glacier ice
C、To present theories of glacier formation
D、To discuss the similarities between glacial types