①将两只空烧杯分别放在天平的左右两盘内，把合金块放入左盘烧杯中；

②向右盘烧杯中缓缓倒水，再用滴管调节，直至天平平衡；

③将烧杯内水倒入空量筒中，测出水的体积（如图a所示）；

④用细线拴好合金块，将其放入图a的量筒内，测出水和合金块的总体积（如图b所示）．则

       If we want to deal with the association (社交) between boys and girls properly, here are some "dos and don'ts" for you to follow.
Keep a normal and healthy state of mind. Our schools and classes are made up of boys and girls. It is very nature for the boys and girls to make friends with each other. We should make as many friends as possible. We should keep touch with the other sex (性别) in public instead of in secret.
      Don't be too nervous or too shy. If you are a shy person, you can also find a way out. First of all, you can make friend with the students who have the same interest and hobby as you. As both of you have much in common, you may have much to talk about. If you keep doing like that, little by little, you will gladly find you are also as free to express yourself as others.
     Don't fall into the ditch of early love. The boys and girls at a adolescence (青春期) are rich in feeling. They  are easy to regard the friendship as a sign of love and fall in love with each other at an early age. In my opinion, early love is a green apple that can't be eaten. An apple won't taste sweet until it is full ripe. Boys and girls at middle school are too young to carry the heavy duty of love. Do keep out of early love. 

分类法是学习化学的有效方法之一．下列各组物质按照混合物、氧化物、单质的顺序排列的是（    ）