复合宾语的被动语态：原来的句子是“主语＋谓语＋宾语＋宾语补足语”,在变被动语态的时候只
能将宾语变为被动语态的主语,宾语补足语变为被动语态的主语补足语.
例如：They heard her sing an English song at the party.
他们在晚会上听到她唱了一首英语歌.
变为被动语态是：She was heard to sing an English song at the party.
What about- They heard her singing an English song at the party.
add your reason