单项选择题
A．only when we cannot see perfectly B．only when we can see well C．even if we can see well D．only when we realize how important our eyes are
A．the trade of farm birds B．the waste and liquids of infected birds C．the fact that sortie people touched sick birds or any infected part of sick birds D．the virus changed and started spreading in some new forms
A．farm birds B．infected farm animals C．people died in Hong Kong D．a different kind of bird flu virus