A thin glass tube 1 m long is filled with Ar gas at 1 atm, and the

Question:

A thin glass tube 1 m long is filled with Ar gas at 1 atm, and the ends are stoppered with cotton plugs:

HCl gas is introduced at one end of the tube, and simultaneously NH3 gas is introduced at the other end. When the two gases diffuse through the cotton plugs down the tube and meet, a white ring appears due to the formation of NH4Cl(s). At which location-a, b, or c-do you expect the ring to form? Explain your choice.