根据短文内容和首字母提示，在下文空格处填入适当的词使短文完整。在标有题号的横线上，完整写出空缺处所填单词的正确形式。
       Remember to be a good boy or a girl any time. It's i____  to have good manners, especially in some p____ places. When you are waiting in line, don't jump the queue. Be p____ . Don’t stand too close to others in the line. Keep 20cm a____  if you can.
     F____  the advice of the workers when your are having a visit on a place of interest. Do not climb on exhibitions. Make s____ it's OK to take pictures before you hold up your camera. When you go to shows, w____  clean clothes. You should get to the show 15 minutes before it starts. During the shows, turn o____ your cell phone. Don't eat during the show. Restaurants will be quite busy. Don't stay too long when you finish eating. Remember to t____ your rubbish away after the meal.
    Good manners can make you a g____ or a lovely lady. Wherever you are, others will be pleased with you.

二氧化硫是污染大气的有害物质之一，下列哪项可作为检测大气污染程度的指示植物（    ）

阅读下文，回答相关问题。
        一屠晚归，担中肉尽，止有剩骨。途中两狼，缀行甚远。
       屠惧，投以骨。一狼得骨止，一狼仍从。复投之，后狼止而前狼又至。骨已尽矣，而两狼之并驱如故。
       屠大窘，恐前后其受敌。顾野有麦场，场主积薪其中，苫蔽成丘。屠乃奔倚其下，驰担持刀。狼不敢前，眈眈相向。
       少时，一狼径去，其一犬坐于前。久之，目似瞑，意暇甚。屠暴起，以刀劈狼首，又数刀毙之。方欲行，转视积薪后，一狼洞其中，意将隧入以攻其后也。身已半入，止露尻尾。屠自后断其股，亦毙之。乃悟前狼假寤，盖以诱敌。
        狼亦黠矣，而顷刻两毙，禽兽之变诈几何哉？止增笑耳。

有X、Y、Z三种金属，如果把X和Y分别放入稀硫酸中，X溶解并产生氢气，Y不反应；如果把Y和Z分别放入硝酸银溶液中，过一会儿，在Y表面有银析出，而Z没有变化．根据以上实验事实，判断这三种金属的活动性由强到弱的顺序是（    ）