18.在正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,点EEE、FFF、GGG分别为棱A1D1A_{1}D_{1}A1D1、A1AA_{1}AA1A、A1B1A_{1}B_{1}A1B1的中点,下列命题中正确的是((( )))