Explanation: Contacts designed to slide against each other during opening and closing are indeed known as **self-wiping** contacts. This sliding motion is crucial because it physically scrapes away microscopic layers of oxidation, dust, and other contaminants that can accumulate on the contact surfaces. This "wiping" action ensures that a fresh, clean metal-to-metal surface is presented for electrical contact each time the switch or relay operates. A clean contact surface minimizes electrical resistance across the connection, preventing heat buildup and ensuring reliable signal transmission, which is vital for proper operation of radio equipment. Butt-joined contacts, conversely, meet head-on without this cleaning action and are more susceptible to poor connections due to surface contamination. Therefore, both descriptions in B and C are accurate, making D the correct answer.