题目

 阅读短文，然后根据要求完成任务。 What do you think is the most important invention? The TV? The printing? The telephone? Or the computer? A report show that they are not the only important invention. Here are three other important inventions.   One of the simplest inventions is the toilet(厕所). Before toilets, waste ran into rivers from people’s houses. Illness were common in big cities, and it was often difficult to find clean drinking water.   Animal food is another important invention. Before the invention, most animals were killed every autumn because there把等边直角尺悬挂在光滑水平轴O处，再在尺上加外力F使尺平衡在图示位置．在下面4种悬挂方式中所加最小外力分别为F1，F2，F3，F4，四个力相比（　　）A．F1最大B．F2最小C．F1和F2相等D．F3和F4相等