In the set of animals, a relation $\mathrm{R}$ is defined in each question.
On the set of human beings a relation $\mathrm{R}$ is defined as follows:
$a \mathrm{R} \mathrm{b}$ if $^{\prime \prime} \mathrm{a}$ and $\mathrm{b}$ have the same brother", then $\mathrm{R}$ is
(1) only reflexive
(2) only symmetric
(3) only transitive
(4) equivalence