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You could end the presentation by saying:

Dyna ddiwedd y cyflwyniad. – That’s the end of the presentation.

You could then thank the audience for listening:

Diolch am wrando mor dda. – Thank you for listening so well.

Diolch am wrando. Gobeithio eich bod chi wedi mwynhau'r cyflwyniad. – Thank you for listening. I hope that you have enjoyed the presentation.

You could also invite the listeners to ask questions.