[R] VLMs Behavior for Long Video Understanding

I have extensively searched on long video understanding datasets such as Video-MME, MLVU, VideoBench, LongVideoBench and etc. What I have seen there these datasets are focused on different categories such dramas, films, TV shows, documentaries where focus on tasks like ordering, counting, reasoning and etc.

I feel that multi-step reasoning is less explored and then what i have did i designed the questions with no options just ground truth and asked the VLM to give me the answer but VLMs unable to give the answer. But when i give the 4 options then VLM achieves 100% accuracy.

My point is that why VLMs behave like this?