5. Which of the factor that does not affect the strength of the electromagnet?
A. number of coils of wire around the nail
B. number of dry cells used
C. number of thumbtacks
D. size of nail
6. What do you need to do in order to pick up more than 2 thumbtacks?
A. wrap a wire around a nail 20 times and attach a battery
B. wrap a wire around a nail 40 times and attach a battery
C. take the battery away
D. take the wire away​