"Frontier", one of many English words that took on new meanings in North America, has assumed as well a role in explaining the continent’s history during the past five hundred years. In time the word has acquired other connotations, both positive and negative. Among historians, the term "frontier" is most closely associated with Frederick Jackson Turner, whose essay profoundly influenced American historiography for forty years after its publication in 1893. Reacting against historians who considered American history essentially an outgrowth of British and European institutions, Turner argued that Old World customs and attitudes broke down and reformed in America’s radically different physical and social environment. The opportunity of "free land" drew pioneers westward into settings that required them to modify or scrap entirely many of the institutions and values of their previous lives. The result was a "merged nationality", a distinctive culture and people. Although he emphasized the positive, Turner observed that the same conditions that had helped reshape the society had less desirable effects. For instance, as early governments they had to create political forms almost on the fly, they were less likely to innovate than to copy what they knew from the past. The tension between change and tradition was played out in gender relations. Frontier conditions often required women to take on roles usually reserved for men, but the crushing load of work made women’s lives difficult and dangerous and left little room for individual fulfillment outside their labors. By Turner’s death in 1932, more fundamental critiques of his ideas were being heard. Some stressed that many other factors—among them patterns of immigration, American society’s middleclass nature, etc.—influenced the national character at least as much as the frontier. Others argued that class divisions and social and economic hierarchies have been much more a part of American life than the frontier-inspired equality implied in Turner’s work. The effect of these various critiques has been paradoxical. No longer considered the primary formative force on continental history, the frontier has been more broadly defined and its explanatory power has grown. Recent research has explored the interactions among Europeans, Euro-Americans, and Indian peoples. Along the various frontiers there developed what the historian Richard White has called a "middle ground", cultures of overlapping customs and mutual borrowing in which all sides created new terms of understanding and exchange. Consequently, many tribes merged and consolidated to meet the threats and opportunities posed by the newcomers. A frontier in this sense was certainly not a division between "civilization and savagery", but rather a place where peoples, ideas, cultures, and institutions came together and interacted on many levels, sometimes mixing and sometimes conflicting but always in mutual influence. According to the last paragraph, frontier was essentially a place of______.

A. overlapping races and institutions
B. interactive groups and values
C. conflicting races and cultures
D. various opportunities and threats