设有关系R和S，关系代数表达式R－（R－S）表示的是：

A、<span style=";font-family:宋体;font-size:14px">R<span style="font-family:宋体">∩</span><span style="font-family:Times New Roman">S </span></span></p><p></p>

B、<span style=";font-family:宋体;font-size:14px">R-S</span></p><p></p>

C、<span style=";font-family:宋体;font-size:14px">R<span style="font-family:宋体">∪</span><span style="font-family:Times New Roman">S</span></span></p><p></p>

D、<span style=";font-family:宋体;font-size:14px">R<span style="font-family:宋体">÷</span><span style="font-family:Times New Roman">S</span></span></p><p></p>