Most animal influenza viruses do not infect humans.Which of the following can explain this observation?

A) Animal viruses fail to destroy ciliated epithelium in that host.
B) Animal viruses do not induce specific cytokines in the host.
C) Animal viruses bind to a different form of sialic acid than human viruses.
D) The HA proteins of animal viruses are activated by the protease furin.
E) Both c and d are correct.