Which of the following is the best example of the law of conservation of mass?

• A
  $12 \ g$ of carbon combines with $32 \ g$ of oxygen to form $44 \ g$ of $CO_2$
• B
  When $12 \ g$ of carbon is heated in a vacuum there is no change in mass
• C
  $A$ sample of air increases in volume when heated at constant pressure but its mass remains unaltered
• D
  The weight of a piece of platinum is the same before and after heating in air