It's important for us to be healthy. But how can we keep healthy? First,you should eat fruit and vegetables three or four times a week. Then eat some tofu every day because it's good for people, both the young and the old. Milk is also necessary(必要的），especially for women. So you should have milk twice a day. It can help you to be strong. Next, doing more exercise is better. You can do morning exercises or evening exercises. After supper you should take a walk.

第一步：取3个装有牛肉汁培养基的培养皿(已高温灭菌)，编号为1、2、3，将标签贴在培养皿的底部；第二步：取1支无菌棉棒，擦拭某同学的手心，打开1号培养皿，将棉棒在培养皿上涂抹，盖好封严；打开2号培养皿，将其暴露在教室空气中约10分钟，盖好封严；3号培养皿不做任何处理。第三步：将1、2、3号培养皿放置在培养箱中培养，每天观察一次，并作好记录。请根据上述实验设计，回答下列问题：