Legal Question in Civil Litigation in California

If you know the answer to a question you are asked, you must provide the truthful answer. If you are not sure of the answer you can answer that you are not sure. If you know the answer and respond by saying "I don't know," you are essentially lying and are subject to penalties for perjury. It is that simple.

Either answer is definitely a lie and subject to a charge of perjury. Also, most liars are not very good at it so it soon becomes obvious that they are not telling the truth and their testimony not only becomes worthless, they often harm the party they are testifying for as the court and jury suspect that party knew the witness was going to lie [which is a separate crime of suborning perjury].