Question

a student wants to test how distance affects magnetic force strength. they perform three tests, but each time they also change which poles face each other. why is the student’s evidence weak?
magnetic force does not change with distance
opposite poles always cancel out results
two variables were changed at the same time
the magnets were not strong enough

Explanation:

In a valid experiment testing one variable (distance on magnetic force), only that variable should change. Here, the student altered both distance and the pole orientation, introducing two changing variables. This makes it impossible to isolate whether changes in force come from distance or pole alignment, weakening the evidence.

Answer:

C. Two variables were changed at the same time