Which of the following sentences is written in passive voice?
A.
Before people could make the sets, someone designed them.
B.
A teacher designed the sets before the students made them.
C.
The sets will have to be designed before they can be made.

C The sets will have to be designed before they can be made.

Explanation: The person or persons who do the action are not the subjects of the clauses. Grammatically, if the verb phrase consists of a form of the verb "to be" followed by the past participle form of the main verb, the sentence or clauses is passive. "...to be designed (by someone)" can be rewritten as "Someone will have to design the sets."

"They can be made by the students." Is passive. Active voice:The students can make them.

im pretty sure a would be the answer