佬____ 蜡____ 惜____
姥____ 腊____ 错____
耐____ 忆____ 损____
奈____ 亿____ 陨____
-- Anna, could you lend me this book, please?
-- I'm sorry. I it for only two days, and I haven't finished it yet.
由A、B两种元素组成的化合物中,A、B两种元素质量比为21:8,A、B相对原子质量比为7:2,则该化合物的化学式为____。