For a real y, let [y] denote the greatest integer less than or equal to y. Then the function  is
 is
 a)    Discontinuous at some x
  b)   Continuous at all x but the derivative  does not exist for some x
 does not exist for some x
  c)     exists for all x but the derivative
 exists for all x but the derivative  does not exist for some x
 does not exist for some x
  d)    exists for all x
 exists for all x