A student moves a horseshoe magnet by hand across a tabletop to varying distances from an electromagnet, as shown in the diagram above. The student records observations of how much force it takes to hold the horseshoe magnet in place at each distance. Observations

• 20 cm: no force required
• 10 cm: small force required; easy to hold in place
• 5 cm: large force required; very difficult to hold in place

What do the student's observations demonstrate?

A. The electromagnet's magnetic field becomes stronger as more magnetic material is added to it.

B. The electromagnet's magnetic field is stronger as more turns are added to the wire on the nail.

C. The electromagnet's magnetic field becomes stronger as a small amount of strength is added to the system by the battery.

D. The electromagnet's magnetic field is stronger close to it than far from it.