Question

They were heard ______ in the next room every night。

A.talking B.to talk

Answer 1

B to talk
根据题意：他们每天晚上都被听到在谈论。

原句：We heard them talk in the next room every night （ heard sb. do sth)
被动语态：They were heard to talk in the next room every night

heard sb. doing sth 是指在某一个时间点发生的事情。

望采纳~

Answer 2

to talk 被动语态中省掉的to要加上，主动句为hear sb do sth ,变为被动句要将省掉的to加上

Answer 3

to talk