How long do you want to live? How many years might be added to a life? Do you want to live another 100 years or more? Some experts say that scientific advances will one day enable humans to last tens of years beyond what is now seen as the natural limit of the human life span.（寿命）
“I think we are knocking at the door of immortality(永生) , " said Michael Zey, a Montclair State University business professor and author of two books on the future. "I think by 2075 we will see it and that's a conservative estimate(保守的估计)."
At the conference in San Francisco, Donald Louria, a professor at New Jersey Medical School in Newark said advances in using genes as well as nanotechnology(纳米技术) make it likely that humans will live in the future beyond what has been possible in the past. "There is a great push so that people can live from 120 to 180 years," he said. "Some have suggested that there is no limit and that people could live to 200 or 300 or 500 years.
However, many scientists who research into ageing are doubtful about it and say the human body is just not designed to last past about 120 years. Even with healthier lifestyles and less disease, they say the failure of the brain and organs will finally lead all humans to death.
Scientists also differ on what kind of life the super aged might live. "It remains to be seen if you pass 120,you know, could you be healthy enough to have good quality of life?" said Leonard Poon, director  of the University of Georgia Gerontology Centre. "At present people who could get to that point are not in good health at all."
By saying "we are knocking at the door of immortality", Michael Zey means ______.

A．they have got some ideas about living forever

B．they are sure to find the truth about long living

C．they believe that there is no limit of living

D．they are able to make people live past the present life span

Donald Louria's attitude towards long living is that ______.

A．people can live from 120 to 180

B．it is possible for humans to live longer in the future

C．the human body is designed to last past about 120 years

D．it is still doubtful how long humans can live

The underlined word "it"( in Paragraph 4)refers to______.

A．using genes as well as nanotechnology

B．the idea of living beyond the present life span

C．a great push

D．the conservative estimate

What would be the best title for this text? 

A．No Limit for Human Life	B．Science, Technology and Long Living
C．Living Longer or Not	D．Healthy Lifestyle and Long Living