B
Oxygen acts as oxidator in the first reaction.
Explanation:
At reactant side:
Mg = 0
At product side:
MgO = 0
but Oxygen (O) = -2
Mg + O = 0
Mg + (-2) = 0
Mg - 2 = 0
Mg = 2
Magnesium is oxidized by OXYGEN from zero to +2
radioactive decay occurs in unstable atomic nuclei β that is, ones that don't have enough binding energy to hold
the nucleus together due to an excess of either protons or neutrons.