A train yard uses an electromagnet on a crane to lift metal debris along the tracks and load it onto rail cars. The electromagnet on the crane can also move blocks of compressed scrap metal from trucks onto rail cars.

Why is an electromagnet the best kind of magnet to use for these tasks?

O A. An electromagnet can pick up more kinds of metal than other types of magnets.
O B. An electromagnet is the only kind of magnet large enough to lift blocks of scrap metal.
O c. An electromagnet has a permanent and stable magnetic field that other magnets do not have.
O D. An electromagnet can be turned on when lifting metal objects and turned off to drop them, unlike other magnets.