据科学家测算，冥王星与太阳的距离大约是五十九亿八千零五十万千米，这个数写作（）千

水（H₂O）、氧气（O₂）和二氧化碳（CO₂），尽管它们的组成和性质不同，但它们的成分中都含有__________元素。

根据句首字母补全单词

1. I have to make a p______for my study.

2. School is closed for the summer and children are on v________.

3. New year is coming. I am going to post a p______ to my friend.

4. Parents must s_______ their children to school.

5. He is a_______ from home for a long time.

烛焰通过凸透镜成像如图所示，利用这一成像规律制造的光学仪器是__________。

Which pen is _____, this one or that one?[ ]

A. cheap                

B. cheaper            

C. cheapest            

D. the cheapest

在进行直流高压试验时，应采用正极性直流电压。（）