题目

．如图，BD是△ABC的角平分线，DE∥BC，交AB于点E，DF∥AB，交BC于点F，当△ABC满足条件________时，四边形BEDF是正方形． 答案：∠ABC＝90° An opinion is _____ someone believes is true but has not been proved.A. that B. whatC. which D. whether