Please answer a question about the following article about The Sun (United Kingdom):

Murdoch has responded to some of the arguments against the newspaper by saying that critics are "snobs" who want to "impose their tastes on everyone else", while MacKenzie claims the same critics are people who, if they ever had a "popular idea", would have to "go and lie down in a dark room for half an hour". Both have pointed to the huge commercial success of the Sun in this period and its establishment as Britain's top-selling newspaper, claiming that they are "giving the public what they want". This conclusion is disputed by critics. John Pilger has said that a late-1970s edition of the Daily Mirror, which replaced the usual celebrity and domestic political news items with an entire issue devoted to his own front-line reporting of the genocide in Pol Pot's Cambodia, not only outsold The Sun on the day it was issued but became the only edition of the Daily Mirror to ever sell every single copy issued throughout the country, something never achieved by The Sun.

What distinction does one single edition of The Daily Mirror hold?
the only edition of the Daily Mirror to ever sell every single copy issued throughout the country