Which of the following sentences is written in passive voice?

Which of the following sentences is written in passive voice?

A. A teacher designed the sets before the students made them.
B. Before people could make the sets, someone designed them.
C. Kara designed the sets before the crew made them.
D. The sets will have to be designed before they can be made.

Answer : 

The following sentence is written in the passive voice: The effects of cyanide are well known.