小明用30N的水平推力使箱子沿水平地板匀速滑动，箱子受到的摩擦力是____ N；在箱子上放一袋面粉后，小明要加大力气才能推动箱子，原因是箱子对地板的____增大，导致摩擦力增大．

阅读理解

    Every country has its favourite food. In some countries, people(人们) eat rice every day. They often eat it with meat, fish and vegetables.       

   Japanese(日本人) eat a lot of fish, and they eat a lot of rice, too. In England(英国), the favourite food is fried fish and French fries. In the USA(美国) people like hamburgers a lot. And they like to have cold drinks. In Canada, people eat bread with butter(黄油) on it. In India, people like to eat hot food. Some people eat only fruit and vegetables. They don't like to eat meat and fish. They think green food is good for them.