Answer:
a. He was considered a genius by Mary.
b. The stairway must have been placed by a trespasser.
c. The car was given to me by my mate.
Explanation:
A verb's voice might be active or passive depending on the kind of action it is doing.
The link between the subject and the verb is clear when using the active voice.
Using passive voice is appropriate when the action, rather than the subject, is the point of the sentence. It doesn't matter who does it.
