The ability to produce billions of different antibodies in humans results from:

A.	The presence of billions of complete antibody genes in B cells
B.	The fact that both T cells and B cells contain antibody genes
C.	The production of variable regions of light and heavy antibody genes by DNA rearragement
D.	The fact that a single antibody gene produces an antibody capable of billions of different three-dimensional structures and the ability to combine with any antigen