Which of the following statements correctly describes a symmetric relation on a non-empty set A?
a) For every a∈A, (a,a)∈R.
b) For all a,b∈A, exactly one of (a,b) or (b,a) belongs to R.
c) For all a,b∈A, if (a,b)∈R then (b,a)∈R.
d) For all a,b∈A, (a,b)∈Ror(b,a)∈R.