A famous actor has been criticised for flying by private jet to environmental conferences where he speaks about the urgent need to reduce carbon emissions. His supporters argue that his celebrity status allows him to reach millions of people with his environmental message, and that this positive impact far outweighs his personal carbon footprint. However, critics maintain that public figures who advocate for behavioural change have a special responsibility to model that behaviour themselves, regardless of how much good their advocacy might do. If people see their environmental champions living unsustainably, they will be less likely to take the message seriously, no matter how eloquently it is delivered. Which one of the following illustrates the principle used in the above argument?