根据短文理解，选择正确答案。

    Frederick was ill and went to the hospital. A doctor looked him over and said, "Well, Mr Green, you're going to get some injections(针), and you'll feel much better. A nurse will come and give you the first this evening, and you'll have to get another one tomorrow evening.'' In the evening a young nurse came to Frederick's bed and said to him. "I'm going to give you your first injection now, Mr Green. Where do you want it?"

    The old man was surprised. He looked at the nurse for a while, then he said, "Nobody has ever let me choose that before. Are you really going to let me choose now?"

    "Yes, Mr Green," the nurse answered. She was in a hurry, "Where do you want it?"

    "Well, then,'' the old man answered with a smile, "l want it in your left arm, please."

 水是生命之源，也是人类最宝贵的资源．请用化学知识回答下列问题：

（1）自来水厂在净水时加入明矾，目的是____．

（2）生活中将硬水软化的一种最常见的方法是____．

（3）欲除去水中不溶性杂质需进行过滤操作，该操作过程中玻璃棒的作用是____．

（4）某同学在完成电解水的实验中，电源正极产生气体的检验方法是____．

（5）节约用水、人人有责．请你提出一条节约用水的合理化建议：____．

一名射击爱好者5次射击的中靶环数如下：6，7，9，8，9，这5个数据的中位数是（   ）