●已知关系R如下表所示，关系R的主属性为__(17)__，候选关键字分别为__(18)__。        R <table style="border: 1pt outset black; width: 31%;" _dj="MsoNormalTable" width="31%" border="1" cellpadding="0" cellspacing="0"><tbody><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; background: none repeat scroll 0% 0% white; height: 12pt;" width="25%">A</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; background: none repeat scroll 0% 0% white; height: 12pt;" width="25%">B</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; background: none repeat scroll 0% 0% white; height: 12pt;" width="25%">C</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; background: none repeat scroll 0% 0% white; height: 12pt;" width="25%">D</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">a</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">b</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">c</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">d</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">a</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">c</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">d</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">e</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">b</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">d</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">e</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">f</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">a</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">d</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">c</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">g</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">b</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">c</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">d</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">g</td></tr><tr style="height: 12pt;"><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">c</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">b</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">e</td><td style="border: 1pt inset black; padding: 0cm; width: 25%; height: 12pt;" width="25%">g</td></tr></tbody></table>