16.??Although American civilization took over and replaced the frontier almost a century ago,the heritage of the frontier is still very much alive in the U.S.today.The idea of the frontier still stirs the emotions and imaginations of the American people.Americans continue to be fascinated by the frontier because it has been a particularly important force in shaping their national values.The frontier experience began when the first colonists settled on the east coast of the continent in the 1600s.It ended about 1890 when the last western lands were settled.The American frontier consisted of the relatively unsettled regions of the country.Here,both land and life were more rugged and primitive than in the more settled eastern part.As one frontier area was settled,people began moving farther west into the next unsettled area.By settling one frontier area after another,Americans moved across an entire continent,2,700 miles wide. 问题：The frontier continues to attract Americans because ( )

A. the movement helped to shape American values

B. American people are bored with the clamorous city life

C. American people are emotional and full of imagination

D. the movement took place a century ago