Context: The Union and Confederacy built many new ships during the Civil War. This is a painting of the first battle between two ironclads, the CSS Virginia and the USS Monitor. Use it to answer the question below.
Based on the painting, which was true about ironclads?
A. They were powered by oars.
B. They didn't have weapons.
C. They were made of metal.
D. They used sails.
Determine the correct option from the provided choices and reply with its corresponding letter.
Answer: C