Question

Which sentence is written in the active voice?

The order was placed yesterday afternoon by the science director's assistant.
The science director's assistant placed the order yesterday afternoon.
Yesterday afternoon, the order was placed by the science director's assistant.
Yesterday afternoon, the placement of the order was done by the science director's assistant.

Answer 1

It's the one that doesn't use a helping verb

Answer 2

Active voice is usually subject-verb-action.
Passive voice is action-helping verb-verb-preposition-su ject