科学家最新研制了一款微型无人侦察机——“蜂鸟”.为了减轻其质量，该侦察机的材料应具有的特点是   （）

    English class is always full of fun. It's a tradition to put on an English play at the end of the year. Yesterday our English teacher gave us copies of the play.

    "Choose a character in the play whom you'd like to be," she told us. "This afternoon you will learn a few lines (台词) of this part for homework. Then tomorrow you can say those lines in front of the class. I'll decide who plays each part."

    I wanted to have one of the biggest parts in the play, so I chose the part of the king. He has a lot of lines to say. I learnt them by heart.

    This morning in our English class we had to say our lines. I acted the part of the king. When l was saying my lines, I put a lot of expressions into my acting. I thought I acted really well.

    Many of the students in my class couldn't act at all. They couldn't even read without making lots of mistakes. I was sure that l would get a big part in the play.

    However, the teacher gave me the part of a soldier. He has only one line. It is, "Yes, sir!"

    I was very disappointed. At the end of the lesson I went up to her and asked her why I had such a small part.

    "You put too many expressions in your acting," she said. "Good acting is not big acting. It is acting the audience can believe in." I think I know what she means.

下面关于空气污染指数的叙述中错误的是（   ）。

将NH₄I（s）置于密闭容器中，在一定温度下发生下列反应：
①NH₄I（s）NH₃（g）＋HI（g）；②2HI（g）H₂（g）＋I₂（g）。
达到平衡时，c（I₂）＝0．4mol·L^－1，c（HI）＝4.2mol·L^－1，则此温度下反应①的平衡常数为（      ）