题目

阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项。Rwanda is a small country in East Africa with about more than 13 million people. The landlocked country is one of the fastest growing economies in Africa. 1, almost half of the population still live in poverty.Many people live in 2 where there is no doctor. Medical supplies such as 3, vaccines (疫苗) or even blood are 4 to reach these areas.Zipline, an American start-up company, now helps people in remote areas to get better 5. A multinational team of engineers ha某同学从五楼的阳如处释放两个易拉罐，一个盛满水，一个是空的，他发现空易拉罐比盛满水的易拉罐晚落地一段时间，他想这一定是空气阻力引起的，于是他设计了一个测定空易罐所受空气阻力的实验，他先测出了空易拉罐的质量m，又测出了一层楼的高度，及五楼地板到阳台窗口的距离，他再请来别一位同学在楼下用秒表测出空易拉罐从五楼阳台窗口到落地所用时间t．根据以上数据，计算空气阻力的表达式为________．