Which of the following would you expect to find in an inducible system?

• A
  $A$ repressor protein,which is bound to $DNA$ in the absence of any other factor.
• B
  $A$ repressor protein,which is bound to $DNA$ in the presence of a corepressor.
• C
  An activator protein,which is bound to $DNA$ in the absence of any other factor.
• D
  An activator protein,which is bound to $DNA$ only in the absence of an inhibitor.