A student made an electromagnet using an iron bolt, copper wire coiled three times around the bolt, and a battery. How can he increase the strength of his magnet?
A Add batteries to increase the voltage, and increase the space between the coils of wire.
B Add batteries to decrease the voltage, and increase the space between the coils of wire.
C Add batteries to increase the voltage, and decrease the space between the coils of wire.
D Add batteries to decrease the voltage, and decrease the space between the coils of wire.