Maria makes an electromagnet shown in the figure below. The battery has a voltage of 6.0V, and she uses an iron nail. Which of the following would increase the strength of the magnetic field produced?
a
Reversing the direction of the current flow.
b
Replacing the iron nail with one made of plastic.
c
Changing the battery to a 3.0V battery.
d
Doubling the number of times the wire wraps around the nail.