Question: There is considerable evidence that the virus that causes COVID-19 arose in non-human mammals, such as pangolins or bats. Which of the following is the MOST PLAUSIBLE explanation for why no birds have been implicated as vectors? Need Assignment Help? Question options: Birds are ectotherms, and the virus requires high temperatures to reproduce. The virus relies on the adaptive immune system, and birds have only an innate immune system. The layer of feathers may prevent this virus from entering across the skin. The proteins that enable the virus to bind to and infect cells may be different in mammals and birds.