According to Professor Erika Simpson's lecture, why did game theoreticians find it challenging to make credible threats under the concept of Mutual Assured Destruction (MAD)? In short, what was the problem with MAD's credibility? A. Doomsday machines were ineffective in practice B. The opponent's rank ordering of choices would always be unknown C. Communication was not considered a key factor D. MAD was based on irrational decision-making E. MAD prioritized the avoidance of any armed conflict including a possible nuclear war