Which of the following statements concerning the maintenance of solid-silver contacts in relays and auxiliary control circuits is correct? A. When necessary, they should always be dressed with a wire wheel. B. They should be filed with a fine-cut file when projections extend beyond the contact surface. C. When black silver oxide is present, it should always be removed from the contact surface with coarse sandpaper. D. If necessary, they should be held together with moderate pressure while emery paper is drawn between the contacts.