Or
B) Get impatient, cut them off and tell them everything you know about the topic?
Or
C) Patiently let them finish, THEN tell them you already know, and everything you know about the topic?
I personally have a tendency to be an impatient douche sometimes so I admit I don’t always handle these situations with the greatest patience & social grace.
This is similar to the guidelines for correcting someone politely. Like if you used a word wrong, instead of saying well actually that word means this, you use the word correctly, you can affirm what they just said but using the word correctly. So you never directly correct them.
It’s the same basic kind of principle.