One metallic sphere $A$ is given a positive charge,whereas another identical metallic sphere $B$ of exactly the same mass as $A$ is given an equal amount of negative charge. Then:

• A
  Mass of $A$ and mass of $B$ still remain equal.
• B
  Mass of $A$ increases.
• C
  Mass of $B$ decreases.
• D
  Mass of $B$ increases.