单选题：A relation $$R$$ is defined on a set $$S$$. If for any elements

A relation $$R$$ is defined on a set $$S$$. If for any elements $$a$$ and $$b$$ in S, we have that if "$$a$$ $$R$$ $$b$$" is true, then "$$b$$ $$R$$ $$a$$" must be true, then $$R$$ is said to be __ over S. @[C](1)

A. reflexive
B. transitive
C. symmetric
D. invertive




A.reflexive
B.transitive
C.symmetric
D.invertive


答案：C