管材挤出的工艺过程是什么？挤出管材如何定径？

用什么方法可以证明普通酒精中含水？

在下列各句的空白处填入一个适当的词，使句子意思完整，语法正确。

1. My daughter bought a new _________ of shoes for me.

2. I haven't _________ your letter yet. When did you send it?

3. I don't like having classes on Saturday. Me __________.

4. The traffic is busy at that time. You'd better be careful to _________ the street.

5. It is better to keep your voice down in _________ places.

螨类的主要特征有（）。

A、体长0.3~0.4mm

B、乳白色半透明

C、成满4对足

D、成螨有1对翅

图书馆事业建设的原则主要有哪些？