As an extinguishing agent，foam（）.

A.conducts electricity

B.should be directed at the base of the fire

C.is most effective on burning gases which are flowing

D.extinguishes by cooling oil fires below ignition temperature

下列软件中，可以免费下载使用，但若正式使用仍需付费的是（）。

A.专利软件

B.公用软件

C.共享软件

D.免费软件