premise: A snowboarder wearing a blue jacket and green pants is speeding down the side of a mountain with snow shooting behind him, with a blue sky and green trees in the background. hypothesis: Two people skiing slowly and cautiously with the dark night sky behind them.
contradiction: The use of 'a' means that there is only one snowboarder, not two. A snowboarder can also be a skier, but cannot do both at the same time. They also cannot be speeding and going slowly at the same time.