设A={a, b, c, d},A上的等价关系R={<a, b>,<b, a>,<c, d>,<d, c>}∪IA,则对应于R的A的划分是( )A、{{a}, {b, c}, {d}}B、{{a, b}, {c}, {d}}C、{{a}, {b}, {c}, {d}}D、{{a, b}, {c, d}}