Toss Up - Biology - Multiple Choice

There are more possible antibodies to antigens than there are genes that code for these antibodies in the human genome. Which of the following BEST accounts for this:
W) genes mutate constantly in B cells
X) genes for antibodies are spliced together from different chromosomal loci
Y) antibodies have large amounts of cross reactivity with various antigens
Z) antibodies are only specific for certain parts of an antigen