19.( )若放電前後分別測定鋅板和銅板的質量,得到結果如下表,則下列關係式何者正確?(原子量:Zn=65.4,Cu=63.5 ) (A) X+Y < Z+W(B) X+Y > Z+W (C)X+Y=Z+W(D)X+Z=Y+W