过D作DF垂直BC于F,设BE=x海里依题:CF=DF=BF=100,所以EF=100-xAB+BE=2*DE所以,DE=100+x/2DE^2=DF^2+EF^2所以,x=400-200√6/3所以,DE=200-100√6/3所以,选B望采纳,谢谢!