Blanking in an Interview - Ask Q’s or admit that you don’t know?

Say you blank on a technical question. Is it best to admit that you don’t know OR does it show more tenacity and potential for “partial credit” if you ask follow up Q’s to understand the question and/or define key terms?

EX: Q: “Which is better: $500 cost synergy or $500 revenue synergy?” A1: “I have no clue. I could guess, but it would be completely random” A2: “Im not sure on this one. My intuition says…bullshit answer” A3: “Hmm.. I’m not sure. Could you help by defining cost synergy and revenue synergy and I could take a stab at walking you through after I know what those terms mean?”