What does the presence of a vestigial structure in a modern organism indicate?
a
The structure was probably functional in some ancestor of the modern organism
b
A limited number of genes are expressed during the lifetime of an organism
c
Natural selection perpares the body of organisms so that in the future the vestigial structures may be used
d
The vestigial structure was not used by the modern organism, so it became nonfunctional within the organism's lifetime