Severe acute respiratory syndrome (sars) is caused by sars-associated coronavirus (sars-cov).  the diagram shows the structure of sars-cov.

sars emerged as an infectious human disease in 2002.  several years of research have led to the conclusion that sars-cov originated in a single colony of bats.  these bats served as a reservoir for a form of the virus which could not infect humans.  sars-cov is hypothesized to have mutated in such a way that it gained the ability to infect humans.
in what way are the glycoproteins on sars-cov involved in its ability to infect humans?
  a. 
the glycoproteins the virus travel in the air so it can spread among humans.
  b. 
the glycoproteins interact with a human cell's dna replication machinery so the virus can replicate.
  c. 
the glycoproteins the virus move around the human respiratory tract so it can damage the lungs.
  d. 
the glycoproteins interact with receptors on a human cell so the virus can enter the cell.