Answer:
1). The students were told the answers.
2). We have been shown the movie by them.
3). Many movies have been made by Steven Spielberg.
4). A type of air pollution is caused by hydrocarbons.
5). The kayak was invented by Indigenous people for transportation and fishing.
6). 350 million letters are sent by people every year in Britain.
7). All the cookies had already been eaten by them when I arrived.
8). The factory was opened by us at nine o'clock.
9). The documents are kept in this drawer.
10). We were told by a man to wait outside.
Explanation:
Passive voice is described as the grammatical construction in which the object and the action stated by the verb are emphasized rather than the subject. The subject in such construction becomes the recipient of the action and is acted upon(by the verb). It helps in showing interest in the noun(in object place) which undergoes the action instead of the doer(subject) who performs it.
In order to convert the given sentences from the active voice to the passive voice, the object will be substituted with the subject in order to shift the emphasis. Since the subject is the recipient of the action, it can be omitted or written using 'by' as mentioned above.
