Very good question.
The error rate so far (86% - 241 out of 280) is amazing. Most of those errors were the same wrong answer, there appears to a widely distributed myth that any primary index is clustered unless it is declared as unclustered. I knew that such a myth existed, but I thought it was so often debunked that only a small proportion of people still believed it. Well, that was clearly wrong - even though some of that 86% probably arises from carelessness in reading the question what is left after allowing for that carelessness is not a small proportion!