When an alkyne, such as but-2-yne, is subjected to D2 and palladium, 2 equivalents of D2 are added. When an alkene, such as cis-but-2-ene, is subjected to these conditions, only 1 equivalent of D2 is added. When an alkane, such as butane is subjected to these conditions, no D2 is added. Write the reactions and explain.