D

    How old is "old"? The answer has changed over the years. Two Hundred years ago, you were old at 35. At the beginning of the 20th century, the average life span(平均寿命) was 45. In 1950, 70-year-olds were really old. Today, a healthy 70-year-old is still thought young.

    So, how old is "old"? The answer is one you've heard many times, from all kinds of people. "You are as old (or young) as you feel. " Your age simply tells you how many years you have lived. Your body tells you how well you've lived.

    "Nobody grows old by living a number of years, "Wrote a writer. "People grow old when they don't have their ideals(理想)." People shouldn't have the wrong ideas about aging. Sometimes, older minds can be as bright as young minds. Alice Brophy once said, "It makes me unhappy when people say, 'You look young for your age.' What does that mean? You know you can die old at 30 and live young at 80."

有一块长方形的铜条，有关于它的三种说法：

①这是一块单晶体，因为它有规则的几何形状．

②这是一块多晶体，因为它内部的分子是不规则的，没有形成点阵结构．

③这是非晶体，因为它的物理性质是各向同性的．

这三种说法中：（   ）

小明为了证明蚯蚓喜欢阴暗的环境，他选择了两条蚯蚓，一条放在光下，另一条放在阴暗处。他的行为属于（   ）