The molar mass of an unknown organic liquid (M ~ 100) is determined by placing 5 mL of the liquid in a weighed 125-mL conical flask fitted with a piece of Al foil with a pin hole in it. The flask is heated in a boiling water bath until the liquid evaporates to expel the air and fill the flask with the unknown vapor at atmospheric pressure. After cooling to room temperature the flask and its contents are reweighed.
The uncertainty in which piece of apparatus causes the largest percentage error in the molar mass?

A) balance (+/-0.01 g)
B) barometer (+/-0.2 mm Hg)
C) flask (+/-1.0 mL)
D) thermometer (+/-0.2 ˚C)