15.如图,在直四棱柱ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,底面ABCDABCDABCD为矩形,AB=2ADAB=\sqrt{2}ADAB=2AD,EEE,FFF分别为BB1BB_{1}BB1,ABABAB的中点,则((( )))