There is a new park near my house. My family often go to the park on the weekend. There aren't any tall buildings in the park. There is a big lake in the park. But we can't swim in the lake. There are many flowers and trees near the lake. There is a small hill behind the lake. Beside the lake, there is a small shop (商店). The park is beautiful. I like it very much.

皮尔•卡丹是世界著名的服装设计师。他从小就对服装感兴趣，然而他的父亲却希望他成为药剂师，结果药剂学学不好的他还是选择了在服装方面发展自己，走上了服装设计的道路，并取得了巨大成功。这个故事告诉我们（    ）

计算a+(－a)的结果是（    ）

现有由同种材料制成的A、B两金属球，其中一个是实心的，它们的质量分别为128g、60g，体积分别为16cm³、12cm³．
（1）通过计算说明，哪一个球是实心的？
（2）这种金属材料的密度是多少？
（3）空心球的空心部分体积是多大？