A post office argues: 'Opening a separate counter for parcels will shorten the queue at the letter counter, because at present everyone stands in the one line whatever business he has come on.' Which of the following, if it turned out to be FALSE, would most damage the argument?

A parcel counter shortens the remaining queue only by drawing people out of it. If parcel senders are a handful, almost everybody stays where he is and the letter queue is barely touched, so this is the claim the argument leans on hardest. Saturday hours, the evening collection van and the number of clerks could each be false while the argument still stood.