well i had second thoughts about pressure, so i wrote what this guy did:Yeah i agree, wouldnt a decrease in pressure result in all substances being reduced by an EQUAL amount? because all gases are 24.79L in STP and each gas CO, Cl2 and COCl2 were all the same amount of moles (1:1:1 ratio) .'. they would all decrease the same amount, what the question was saying was that each was different, hence they it cant be reduction in pressure.
so for the last part where they all dropped is it valid to say that an amount of gases was removed from the system
but yeah.... who knows.