A student has a small metal sphere that hangs from an insulating string. The student, in seeking to determine the charge on the sphere, brings a positively charged rod near the sphere and finds that the sphere is attracted to the rod. The student then brings a negatively charged rod near the sphere, and finds that the sphere is again attracted to the rod. Which of the following conclusions, if any, can be drawn from these results? a. The sphere must be negatively charged.
b. The sphere must be positively charged.
c. The sphere must be uncharged.
d. No conclusion can be made about the charge of the sphere.