如图，已知AB=12；AB⊥BC于B，AB⊥AD于A，AD=5，BC=10．点E是CD的中点，则AE的长是（）。
参考答案：
Having checked the doors were closed, and ________ all the lights were off, the
boy opened the door to his bedroom.
A．why
B．that
C．when
D．where
化学事故有（）的特点。
A、突发性强，难以预防
B、扩散快，受害面广
C、对人员伤害途径多，形式特殊
D、续时间长，消除困难
