Select the correct active form of the given sentence.
The main gate of the building was being guarded by gun-totting guards.

A

Gun-totting guards guarded the main gate of the building.

B

The main gate of the building were guarding gun-totting guards.

C

Gun-totting guards were guarding the main gate of the building.

D

Gun-totting guards have been guarding the main gate of the building.