5.正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,PPP,QQQ,RRR分别是A1D1A_{1}D_{1}A1D1,C1D1C_{1}D_{1}C1D1,AA1AA_{1}AA1的中点.那么过PPP,QQQ,RRR三点的截面图形是((( )))