在图中画出光经凸透镜折射后的径迹.
(1)过光心的光线,经过凸透镜其传播方向不改变,如图所示:
(2)平行主光轴的光线,经过凸透镜过焦点,如图所示:
True friendship is like sound health, ________ the value is seldom known until it is lost.
A.from which
B.which
C.of which
D.whose
天然橡胶的化学组成是( )。
A.聚异戊二烯 B.聚甲基丙烯酸甲酯C.聚碳酸酯 D.聚酰胺