A:纯净水
B:牛奶
C:苹果
D:面包
The Great Wall is one of ______ wonders (奇迹)in the world.
如图是光学显微镜的4个镜头,甲乙一端无螺纹,丙丁有螺纹.若要在视野内看到最多的细胞,宜选用的镜头组合是( )