The best material for the core of the primary and secondary coils of an induction coil is

• A.copper, because it has a good conductor of electricity
• B.steel, because it becomes magnetic and retains its magnetism for a long time
• C.rubber, because it is a good insulator and the user is prevented from shock
• D.soft iron, because it becomes magnetic but easily loses its magnetism
• E.glass, because it is transparent