Nearly all of today's Native Americans in North, Central and South America can trace their ancestry to just six women whose descendants immigrated around 20,000 years ago, a DNA study suggests.          The finding does not mean that only these six women gave rise to migrants who crossed into North America from Asia in the earliest population of the continent.Rather, it suggests that only six left a particular DNA legacy that persists to today in about 95% of Native Americans, said study co-author Ugo Perego in Utah.          "The women did not necessarily arrive together, nor even all live at the same time," he said.Results indicate the women arrived sometime between 18,000 and 21,000 years ago.          The work was published this week by the journal PLoS One.Perego is from the Sorenson Molecular Genealogy Foundation in Salt Lake City and the University of Pavia in Italy.The work confirms the previous indications of just six maternal lineages, as well as a date of around 20,000 years ago when the first people in North America arrived after crossing a land bridge from Asia, Perego said.          The researchers studied mitochondrial   DNA, which is passed only from mother to daughter.They created a "family tree" that traces the different DNA  _ found in today's Native Americans.By noting mutations in each branch and applying a formula for how often such mutations arise, they calculated how old each branch was.That indicated when each branch arose in a single woman.          The six "founding mothers" obviously did not live in Asia because the DNA signatures they left behind are not found there, Perego said.So they probably lived in Beringia, the now-submerged land bridge that stretched to North America, he said. What didn't Ugo Perego and his team do?
They used a "family tree" to analyze the findings. They focused on life styles of the original women. They clarified genetic changes in the family tree. They studied how long each branch had developed.
Question: Nearly all of today's Native Americans in North, Central and South America can trace their ancestry to just six women whose descendants immigrated around 20,000 years ago, a DNA study suggests.          The finding does not mean that only these six women gave rise to migrants who crossed into North America from Asia in the earliest population of the continent.Rather, it suggests that only six left a particular DNA legacy that persists to today in about 95% of Native Americans, said study co-author Ugo Perego in Utah.          "The women did not necessarily arrive together, nor even all live at the same time," he said.Results indicate the women arrived sometime between 18,000 and 21,000 years ago.          The work was published this week by the journal PLoS One.Perego is from the Sorenson Molecular Genealogy Foundation in Salt Lake City and the University of Pavia in Italy.The work confirms the previous indications of just six maternal lineages, as well as a date of around 20,000 years ago when the first people in North America arrived after crossing a land bridge from Asia, Perego said.          The researchers studied mitochondrial   DNA, which is passed only from mother to daughter.They created a "family tree" that traces the different DNA  _ found in today's Native Americans.By noting mutations in each branch and applying a formula for how often such mutations arise, they calculated how old each branch was.That indicated when each branch arose in a single woman.          The six "founding mothers" obviously did not live in Asia because the DNA signatures they left behind are not found there, Perego said.So they probably lived in Beringia, the now-submerged land bridge that stretched to North America, he said. What didn't Ugo Perego and his team do?
A. They used a "family tree" to analyze the findings.
B. They focused on life styles of the original women.
C. They clarified genetic changes in the family tree.
D. They studied how long each branch had developed.
Answer: