it's a difficult thing to plan out. you definitely don't want your answer to sound rehearsed, as you really want conversation to flow naturally. you need to appear more sincere than programmed. acknowledge the validity of the question/concern (if it's posed as concern). provide as objective a reason as possible as to why it turned out that way (did not study enough, did not have a grasp on the material, underestimated the breadth of the material, etc.). come up with a specific example of improvement since then in a field/class as close to that one as possible. mention some greater goal you now have that ensures you wouldn't handle a tough class like that now. affirm that this situation turned out to be a great learning experience. be enthusiastic about all of these points.
i think it's ok to seem as tho you've thot a bit abt this type of question, since you really need to have thot abt your performance in such cases. in this way, it's a fair question, so treat it as such. my conclusion after successfully weathering these types of questions is that you can handle it well enough that no matter what the original concern was, your iv'er is entirely reassured by your response.
even if you can't do all of these things fluently and confidently enough to nail them all, you could hit upon of a few of them strongly to ensure the direction of the answer is positive for your iv'er. the best way to do this is to begin consciously thinking about any issues of your app that could fall into this category in these ways during your free time. thereby, you'll be prepared for any potential iv questions along these lines. good luck.
