题目

I’m usually fairly skeptical about any research that concludes that people are either happier or unhappier or more or less certain of themselves than they were 50 years ago. While any of these statements might be true, they are practically impossible to prove scientifically. Still, I was struck by a report which concluded that today’s children are significantly more anxious than children in the 1950s. In fact, the analysis showed, normal children aged 9 to 17 exhibit a higher level of anxiety today than children who were treated for mental illness 50 yeas ago. Why are America’s kids so s固体物质受热变成气体，这种变化 [　　] A．一定是物理变化 B．一定是化学变化 C．可能是物理变化，也可能是化学变化 D．既不是物理变化，也不是化学变化