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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. 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. 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 nanote物体b在力F作用下将物体a向光滑的竖直墙壁挤压，如图所示，a处于静止状态，（B   ）A．a受到的摩擦力有二个            B．a受到的摩擦力大小不随F变化C．a受到的摩擦力大小随F的增大而增大 D．a受到的摩擦力方向始终竖直向上