Either A or b could be the answer
There is no known beginning so they cant see how it started to form or how to cure it, the virus is also very difficult to predict what it is going to do, it is similar to cancer it could sit around and do nothing or one day it could go wild and kill somone
As a person ages, his or her levels of cardiorespiratory fitness tend to drop the reason being that body slowly decays and does not function the same as it use to when for example you were 25 years old.
I hope it helps, Regards.
<span>Between ages 2 and 6, approximately 45 new epiphyses, or growth centers in which cartilage hardens into bone, emerge in various parts of the skeleton. pg.167</span>