What makes electromagnets more useful that permanent magnets for sorting metals in recycling centers? A. An electromagnet can separate iron from other metals, but a permanent magnet attracts all types of metal. O B. An electromagnet is always "on" but a permanent magnet has to be turned on and off. C. An electromagnet also attracts metals other than iron, but a permanent magnet attracts only iron. D. An electromagnet can be turned on and off, but a permanent magnet is always "on."