A Reaction Vessel Contains Equal Masses of Solid Magnesium Metal

A reaction vessel contains equal masses of solid magnesium metal and oxygen gas. The mixture is ignited and burns with a burst of light and heat, producing solid MgO. The mass of the MgO is less than the initial mass of the magnesium and oxygen. What is your explanation for this apparent loss of mass?

A) Conservation of mass is violated in this reaction.
B) Some of the mass was converted into energy (heat and light) as E = mc².
C) Not all of the oxygen reacted.
D) Not all of the magnesium reacted.
E) Measurement must be in error because mass is conserved in chemical reactions.