Why Would Gene Duplication Events, Such as Those Seen in the Hox

Why would gene duplication events, such as those seen in the Hox gene complex, set the stage for adaptive radiation?

A) Without duplicated genes, species would be vulnerable to extinction.
B) The original gene copy is the out-group, and the new gene copies are the adaptive radiation.
C) There are more copies of genes, meaning speciation had occurred by polyploidy.
D) One copy of a gene can perform the original function while other copies are available to take on new functions.