It was sunny and ____ again at this time of year. John went to the beach to ____ his weekend. He was lying on the sand and ____to music. Suddenly he ____ someone crying for help. He ____ up hurriedly and looked towards the voice. He saw a little boy waving(挥动) his hands in the____a little far away. He ____ off his clothes and jumped into the water____. He swam to the little boy, held him, kept his head ____ the water and swam back to the beach. At last, the boy was safe. He thanked John a____.

镁在氧气中燃烧的反应中，镁、氧气、氧化镁的质量比为（　　）