How can you increase the strength of a magnetic field inside a solenoid? a. increase the number of coils per unit length c. place an iron rod inside the solenoid b. increase the current d. all of the above

All of the above foo. Increasing the number of coils allows more current to flow in a unit length with means more magnetic field. The iron rod will be magnetized by the magnetic field of solenoid and will then increase strength of magnetic field. And of course magnetic field increases when current increases

a and b sometimes have a positive value

come closer and become far apart and become steam/gas