By about A. D. 500 the Mound Builder culture was declining, perhaps because of attacks from other tribes or perhaps because of severe climatic changes that undermined agriculture. To the west another culture, based on intensive agriculture, was beginning to flourish. Its center was beneath present day St. Louis, and it radiated out to enclose most of the Mississippi watershed, from Wisconsin to Louisiana and Oklahoma to Tennessee. Thousands of villages were included in its orbit. By about A. D. 700 this Mississippian culture, as it is known to archaeologists, began to send its influence eastward to transform. the life or most of the less technologically advanced woodland tribes. Like the Mound Builders of the Ohio region, these tribes, probably influenced by Meso-American cultures through trade and commerce, built gigantic mounds as burial and ceremonial places. The largest of them, rising in four terraces to a height of one hundred feet, has a rectangular base of nearly fifteen acres, larger than that of the Great Pyramid of Egypt. Built between A. D. 900 and 1100, this huge earthwork faces the site of a palisaded(用栅栏围起的)Indian city which contained more than one hundred small artificial mounds marking burial sites. Spread among them was a vast settlement containing some 30,000 people by current estimations. The finely crafted ornaments and tools recovered at Ca-hokia, as this center of Mississippi culture is called, include elaborate ceramics(陶瓷制品), finely sculpted stonework, carefully embossed and engraved copper and mica(云母)sheets, and one funeral blanket fashioned from 12,000 shell beads. They indicate that Cahokia, was a true urban center, with clustered housing, markets, and specialists in tool-making, hide-dressing patting, jewelry-making, weaving, and salt-making.

What is the main topic of the passage? ______

A．The Mississippian culture.

B．The decline of Mound Builder culture.

C．The architecture of Meso-American Indians.

D．The eastern woodlands tribes.