设A是m×n矩阵，已知 Ax = 0只有零解，则以下结论正确的是（      ）

A、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">&nbsp;</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">m</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">≥</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">n</span></em></p><p></p>

B、<em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">Ax</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">=</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">b</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">&nbsp;<span style="FONT-FAMILY: 宋体">必无解</span></span>

C、<em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">R</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">（</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">A</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px"><span style="FONT-FAMILY: 宋体">）</span>=</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">m</span></em></p><p></p>

D、<em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">n</span></em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">≥</span><em><span style="FONT-FAMILY: 宋体; FONT-SIZE: 14px">m</span></em>