Which change would result in a stronger electromagnet?

Responses

AReducing the number of batteries
Reducing the number of batteries
BIncreasing the voltage of the battery
Increasing the voltage of the battery
CSubstituting another metal for the iron core
Substituting another metal for the iron core
DDecreasing the number of coils around the iron core.

has to be one of the ansers above

Increasing the voltage of the battery