Six students were asked to perform an experiment to see how the mass of a pendulum affected its period. They were given supplies and asked to design an experiment to test the impact that the mass of the bob had on period. What would likely make their results more reliable?

A) If each student was given different equipment.
B) If a set procedure was followed by all of the students.
C) If each student was given the same hypothesis to prove.
D) If the student all perform the investigation on the same day.