对于集合A={1,2,3,4}上的下列关系，是偏序关系的是?

A、R={(1,1),(1,2),(1,3),(1,4),(2,2),(2,3),(2,4),(3,3),(3,4),(4,4)}，;
B、R={(1,1),(1,2),(1,3),(1,4),(2,2),(2,1),(2,4),(3,1),(3,4),(4,4)}，;
C、R={(1,1),(1,2),(1,3),(1,4),(2,2), (2,1),(3,1),(3,3),(4,1),(4,4)}，;
D、R={(2,1),(1,2),(1,3),(1,4),(2,2), (4,3),(2,4),(3,3),(3,4),(4,4)}