If you had added 15.9 mL of 2-butanone (molecular weight 72.11 g/mol, density 0.805 g/mL) to a 50 mL round-bottom flask, how many moles of 2-butanone would you have used?

a) 15.9 / 72.11
b) 15.9 / 0.805
c) 72.11 / 15.9
d) 0.805 / 15.9