The earth's crust averages 16 miles thick and rests on what is called the mantle. This layer, about 750 miles thick, is believed to be of a semi-viscous or plastic nature rather than solid. Thus, both continental and oceanic areas could be said to float on the mantle, with continental areas pushing deeper into the mantle than do oceanic areas where the crust is thinner. Constant erosion moves dirt and rock into ocean areas where they settle. These areas gradually sink deeper into the mantle, eventually creating pressures that thrust new land up from surrounding areas. Mountains formed by this process are made of sedimentary rock and are called folded or dome. Earthquakes occur where the crust shifts or breaks apart sharply under pressure; the resulting breaks, or fissures, are called faults. Mountains created by these faults are called fault blocks. In weak spots on the earth's crust – generally near sea coasts – gases and molten rock under tremendous pressure break through to form conical shaped mountains. These we know as volcanoes. No sooner are new mountains and flatlands created by pressure than water, ice, and wind begin to wear them away, thus creating new valleys, river courses, and coastlines. Read the story below. Select the best answer for the following item. 7. The first paragraph is mainly about the earth's A.)continents B.) crust C.) mantle D.) oceans