题目

根据短文内容从A、B、C、D四个选项中选出一个能填入相应空格内的最佳答案。 Dr. Jack Tod is very interested in cleaning water.  1 . So it’s natural that he is to develop a small and affordable system(系统)to clean waste water. His “Living Machine” can clean waste water in your home or in your business.  2 .The waste water goes into a big plastic tank where bacteria(细菌)start to break down the waste. A few days later after it is dealt with, the water is brought into a greenhouse filled with plants and fish.  3 , the plants and animals remove(去如图所示，用50N力把一个重40N物体压在竖直的墙壁上使其静止，请画出物体所受摩擦力和墙壁所受压力的示意图。