Which of the following sentences uses the active voice?
A. Now just a beacon to history, the old lighthouse saved thousands of lives in its day.
B. Many old lighthouses have been turned into museums or hostels.
C. The lighthouse was built halfway down the cliff so that the beams could be seen below the high fog.
D. During bad weather, keepers would work around the clock to make sure that the light was burning and the lenses were clean.