Why is it important not to present a biased argument as a public speaker?

Because it is immoral

Explanation:

As a public speaker, you most likely have the power to influence other people's thoughts and, just as any other power that one might have, it should be used with responsibility.

By just showing one side, not only you're clearly trying to manipulate other people's thoughts, you're also robbing them the chance of coming to their own conclusion about a subject.

According to the Cambridge Dictionary, something immoral is something that is: "morally wrong, or outside society's standards of acceptable, honest, and moral behaviour"

d)to gain support for the independence movement.

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