回答下列问题：

实验课上，为检验镁条能否在N₂中燃烧，某兴趣小组开展如下探究：

【查阅资料】氮化镁（Mg₃N₂）是一种淡黄色固体，能和水反应，生成Mg(OH)₂和NH₃；

【实验步骤】

①将足量白磷在空气中点燃后，迅速伸入钟罩内，塞紧橡皮塞（图1）；

②待钟罩内水面不再变化时，向水槽中加水，使水槽中液面与钟罩内液面相平；

③打开橡皮塞，将燃着的螺旋状镁条迅速伸入钟罩内，塞紧橡皮塞（图2）；

④待钟罩内水面不再变化时，观察钟罩内壁及表面皿中固体颜色，取少量固体加入水中。

    I saw an accident on my way to the college three days ago.

    I was waiting for the bus at the but stop. When the bus came, I got on the bus and sat on the window side. The bus started to move. After about five minutes, the bus stopped to wait for the signal(信号灯). I looked outside and saw an old man crossing the road. There were not many people on the road. A boy was riding a motorbike very fast form my side. It seemed that he did not see the signal. When the boy took notice of the old man, he tried to slow down his motorbike but there was no time. He collided with the old man very badly. The old man was thrown far away The boy also fell down. His head hit the road but it was saved because of his helmet(头盔). Both of them lay in the pool of blood. Many people gathered(聚集) around them and gave their ideas. I got afraid because I saw the whole accident. Then a woman stopped and called the ambulance( 救护车 ). It came quickly and rushed them to hospital.

    The next day I read the newspaper. The old man died in the ambulance but the boy was saved in time. This was the most terrible accident I saw ever in my life.