如：he was asked to sing a song.
He was kept waiting a long time.
He was made heard by all the students.
认真答题，确保正确，欢迎追问，放心采纳。
追问_____ Of the burden of ice,the ballon climbed up and drifted to the South.
为什么用freed 不用to free
追答这里free作动词，使自由，解放而逻辑主语是句子的主语balloon , 所以是被动。
其实你这句若不考虑意思，单从语法考虑也可以用形容词free
感谢网友们的采纳。
楼主意下如何?
谢谢好评