下列各句中，语气最委婉的一句是(　　)

欧洲流经国家最多的河流是（　　）

C

    Waste can be seen everywhere in the school. Some students ask for more food than they can eat and others often forget to turn off the lights when they leave the classroom. They say they can afford these things. But I don't agree with them.

    Waste can bring a lot of problems. Although China is rich in some resources, we are short of others, for example, fresh water. It is reported that we will have no coal or oil to use in 100 years. So if we go on wasting our resources, what can we use in the future and where can we move? Think about it. I think we should say no to the students who waste things every day. Everybody should stop wasting as soon as possible.

    In our everyday life, we can do many things to prevent waste from happening, for example, turn off the water taps when we finish washing, turn off the lights when we leave the classroom, try not to order more food than we need, and so on. Little by little, everything will be changed. Waste can be stopped one day, if we do our best.

甲、乙两企业是谁违背了买卖合同？为什么？