Which of the following statements regarding transition elements is incorrect?

• A
  For the first five transition elements ($Sc$ to $Mn$),all $4s$ and $3d$ electrons are used for bonding in their highest oxidation states.
• B
  As the number of $3d$ electrons increases,the tendency to participate in bonding decreases.
• C
  Transition elements can exhibit zero oxidation state in complexes.
• D
  In their highest oxidation states,transition metals exhibit basic properties and form ionic compounds.