(08年上海理综)有同学这样探究太阳的密度:正午时分让太阳光垂直照射一个当中有小孔的黑纸板,接收屏上出现一个小圆斑;测量小圆斑的直径和黑纸板到接收屏的距离,可大致推出太阳直径。他掌握的数据是:太阳光传到地球所需的时间、地球的公转周期、万有引力恒量;在最终得出太阳密度的过程中,他用到的物理规律是小孔成像规律和( )A.牛顿第二定律 B.万有引力定律C.万有引力定律、 Teenagers have reached a point in their life_____they are supposed to make decisions of their own for the first time. A. which B. where C. how D. why