Suppose U $=\{2,4,6,$ a$,$ b$,$ c$,\{$b$,$ c$\}\}$ is a universal set with the following subsets:

A $=\{6,$ b$,$ c$,\{$b$,$ c$\}\}$ and B $=\{2,6,$ b$,$ c$\}.$

Which one of the following relations from B to A is NOT functional?

Select one:

a$.$

$\{(2,\{$b$,$ c$\}),($b$,$ c$),(6,6)\}$

b$.$

$\{(2,\{$b$,$ c$\}),($b$,$ b$),($c$,$ b$),(6,$ c$)\}$

c$.$

$\{(2,\{$b$,$ c$\}),(6,$ b$)\}$

d$.$

$\{(6,\{$b$,$ c$\}),(6,$ c$),(6,$ b$),(6,6)\}$