用所给单词的适当形式完成句子。

     __1__  (ask) for information or help is a very common and necessary activity, __2__ (especial) when

 we visit aforeign country. So __3__ (know) how to ask for information __4__ (polite) is important. It's

 important __5__ ( use) correct language.

甲状旁腺素的生理作用是

A．升高血钙

B．降低血钙

C．升高血钾

D．降低血钾

E．升高血钠