Why do you need to weigh your sample multiple times?
A) The hot crucible weighs different than the cool crucible so we need to have multiple measurements to take the average.
B) The magnesium is reacting with oxygen gas in the air. If we weigh the sample and it's different than before, then we know it must be done reacting now.
C) The magnesium is reacting with oxygen gas in the air. If we weigh the sample and it's different than before, then the reaction isn't finished yet.
D) All of the above.