All of the following are reasons why an HIV vaccine has not been developed EXCEPT
A)there is no known animal model of natural immunity for HIV infection. B)use of a weakened virus would potentially transmit the infection. C)small,inexpensive animals which could be used for HIV vaccine research are not available. D)vaccines are not effective against viral infections. E)HIV has a high mutation rate so one vaccination would not be effective to treat the world-wide pandemic.