A:looking, being built
B:looked, being built
C:being looked, building
D:looking, built
①接触面所受的压力大小;
②接触面的粗糙程度;
③接触面积的大小.
通过如图所示实验操作验证这些猜想:
池塘就是一个生态系统,它的组成包括
无公害蔬菜的生产属于( )