B, the opposing forces are the same, thus, the ball doesn't move back or forward.
4. Isotopes of the same element
Explanation:
Student A would be introducing more error. While student B is subtracting the actual filter paper used in the experiment, Student A is using the mass of a different filter paper, which may have a slightly different mass than the filter paper that was used. To avoid having to aacount for this, or introducing such an error, follow Student B in weighing the filter paper before adding the sand.
I hope this helps.
