Helium Effuses Through a Small Opening at a Rate of 1´10^-9

Multiple Choice

Helium effuses through a small opening at a rate of 1´10^-9 mol s^-1. An unknown gas at the same temperature and pressure is found to effuse through the same opening at a rate of 3.78´10^-10 mol s^-1. What is the molecular mass of the unknown gas?

A) 4 g mol^-1
B) 14 g mol^-1
C) 18 g mol^-1
D) 28 g mol^-1
E) 40 g mol^-1