对于集合A={1,2,3,4}上的下列关系,是偏序关系的是?(5.0)
选项：

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)}