21. 如图,在正四棱柱ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,AA1=2ABAA_{1}=2ABAA1=2AB,EEE,FFF,NNN分别是棱CC′CC_{\prime }CC′,C1D1C_{1}D_{1}C1D1,AA1AA_{1}AA1的中点,则((( )))