阅读下列短文，从每题所给的四个选项（A、B、C和D）中选出最佳选项。
D
Jack is a 13-year-old boy.
One day, his father asks him, "How many letters are there in the alphabet(字母表)?"
"24." He says.
"24?" His father asks loudly, "Are you sure?"
"Sorry, I don't know." Answers the son.
"You don't know? You are in school for 7 years and you don't know how many letters there are in the alphabet?" The father is very angry(生气的).
His son says, "No. But let me ask you a question, dad. You often go to the post office(邮局). Please tell me how many letters there are in the post office."
镁、铝、铜三种金属粉末混合物，加入过量盐酸充分反应，过滤后向滤液中加入过量烧碱溶液，再过滤，滤液中大量存在的离子有