解析：选C。考查过去分词作状语。句意：被加热时，液体可以变成气体。由于对于液体来说是加热的承受者，是被动的，因而选C。A项表示主动，B项表示目的，D项是谓语动词的形式。

　　15.Unless ________ to speak，you should remain silent at the conference.

　　A.invited B.inviting

　　C.being invited D.having invited

　　解析：选A。考查过去分词作状语。句意：除非受到邀请讲话，否则在大会上你应该保持沉默。invite与you之间是被动关系，排除B、D。being invited表示正在受到邀请，不符合语境。Ⅱ.句型转换(把下列句子转变成过去分词作状语的结构)

　　1.If I am offered great help，I can get rid of all the difficulties.

　　________ ________ ________，I can get rid of all the difficulties.

　　答案：Offered great help

　　2.Because she was frightened，she asked me to go with her.

　　________，she asked me to go with her.

　　答案：Frightened

　　3.If he is given time，he’ll make a first?class tennis player.________________，he’ll make a first?class tennis player.

　　答案：Given time

　　4.He stood there silently，and he was moved to tears.

　　He stood there silently，________________________.

　　答案：moved to tears

　　5.When they were first introduced to the market，these products enjoyed great success.

　　When first ________________________________，these products enjoyed great success.

　　答案：introduced to the market