(     )27． What does the underlined word "permanent" mean?

根据所给的全部信息写一篇小短文，80词左右。

信息：1．有许多朋友，其中有一个珍视的朋友，从上学至今一直为友。

2．外表(in appearance)严肃（serious）不失趣味，有时甚至快乐，最博识(most knowledgeable)。

3．既是老师也是朋友，无论何时，他陪伴我。

50．1 went to Beijing，but my parents__________ (remain) at home．

49． We__________ (tell) to close the windows before we left the lab．

48．A talk on developments in science and technology__________ (give)in the school next week．

47． Silk__________ (feel) soft and cool．

46． Which Is__________ (difficult) ，part l or part 27

45．I noticed him __________ (go) into the teachers' office just now．

44． The machine was used for__________ (cut) the metal．

43． Tea__________ (bring) to the western world until 1610．

40． What is the most helpful invention? Do you think?（合并为一句）

What__________ __________ __________ __________the most useful invention?