A student measures how many iron pins are lifted by an electromagnet when the power supply is turned on. She then measures how many pins are lifted when the power supply is turned off. She carries out the experiment again with cores made from different materials. The outcomes are shown. Which core is made of soft iron?
- Apins picked up with the power supply on $= 0$, pins picked up with the power supply off $= 0$
- Bpins picked up with the power supply on $= 2$, pins picked up with the power supply off $= 7$
- Cpins picked up with the power supply on $= 8$, pins picked up with the power supply off $= 5$
- Dpins picked up with the power supply on $= 12$, pins picked up with the power supply off $= 0$