Isabella makes an electromagnet by wrapping copper wire around an iron bolt and then connecting each end of the wire to a battery. She observes that the more times that she wraps the copper wire around the bolt, the more paper clips her magnet can pick up. What does she conclude in this experiment?

A
Wrapping more copper wire around the bolt increases the length of the bolt.

B
Wrapping more copper wire around the bolt increases the density of copper.

C
Wrapping more copper wire around the bolt increases the mass of the battery.

D
Wrapping more copper wire around the bolt increases the strength of her electromagnet.