When D2 reacts with ethylene in the presence of a finely divided catalyst, ethane with two deuteriums, CH2D-CH2D, is formed. (Deuterium, D, is an isotope of hydrogen of mass 2.) Very little ethane forms in which two deuteriums are bound to one carbon (for example, CH3-CHD2). Use the sequence of steps involved in the reaction to explain why this is so.