【单选题】

Many AIDS experts are not completely satisfied with results of the study because ______.
A．only a limited number of chimpanzees are used for sampling the virus.

B．it is now extremely difficult to find chimpanzees that carry the virus.
C．the samples collected are from two different subspecies of chimpanzees.
D．it does not provide reliable evidence of the link between SIV and HIV - 1.