A reaction occurs in which carbon combines with oxygen to form carbon dioxide. Is this a chemical reaction or a nuclear reaction, and how do you know? A. This is a nuclear reaction, because the atoms' nuclei changed. O B. This is a nuclear reaction, because mass was conserved. O C. This is a chemical reaction, because only the electrons were rearranged. O D. This is a chemical reaction, because some of the mass was not conserved.