根据短文内容，选择最佳答案。

    Have you ever complained that life is so tiring or that there is too much homework? Does the sky sometimes seem dark to you? Are your studies sometimes not successful? Well, friends, cheer up and smile all the time. If you look at the world with a warm heart, you will find that the whole world smiles at you.
    While in school, sometimes you are tired of your lessons, but have you noticed the happy smile on your teacher’s face when you do a good job?
    If one day it is fine, but just before you go out, it suddenly starts to rain. Maybe you would feel very sad and start complaining about the weather. Dear friends, why don’t you sit down and listen to the free concert that the nature offers you? Also with the timely rain, crops in the fields will grow better and better and farmers will have a good harvest.
    Although everyone wants to succeed in what he tries to do, sometimes failure can’t be avoided. I think failure is not terrible. The terrible thing is that we are afraid of it and give up hope. When we face failure, we must be confident of ourselves, draw a useful lesson from it and try our best to finish what we have decided to do. As a popular saying goes, “Failure is the mother of the success.”
    Attitude decides everything, with an optimistic attitude life is easy and pleasant. Let’s smile at whatever we meet and the whole world will smiles at us.

古代人类钻木取火，这是利用____的方式改变物体内能的．用热水袋取暖，这是利用____的方式改变物体内能的．

如图所示的光路示意图中，MN是空气与玻璃的分界面，PQ为法线，根据图示，MN下方的物质是____ ，折射角是____ ，反射角是____ .