A student is investigating the strength of electromagnets she created using batteries, wire, and nails. (See picture on top) She uses two different-sized nails and varies the number of coils of wire around the nail to determine how many paper clips each configuration can lift. She makes a data table of the results. (See picture on bottom) The student hypothesizes that the number of coils has a cause-and-effect relationship on the strength of the magnetic devices she made. Which would be the best way for the student to test the devices to be sure that her hypothesis is accurate