Which of the following does not describe a safety mechanism to ensure that only infected cells are ...

Which of the following does not describe a safety mechanism to ensure that only infected cells are attacked by NK cells?
 
  a. The default state is one of active inhibition, which must be overcome by activating signals before killing occurs.
  b. Intimate contact with target cells is required.
  c. Activating receptors are induced only after encountering an infected cell.
  d. No single receptorligand interaction induces cytotoxicity, but instead many combinations of receptorligand interactions influence the decision to kill or not to kill a target cell.