The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

Strong earthquakes forced the bottom of the sea to rise above the water. After millions of years, it was pushed up into the air to form land and mountains. Meanwhile, hot liquid rock from deep in the Earth pushed to the surface. This liquid rock slowly cooled. The then cooling liquid formed a very hard rock known as granite.
Theory about （） of land and mountains.