A sedentary lifestyle is typically characterized by:
A. Slow metabolism
However, B. Weight gain can also be a common consequence of a sedentary lifestyle, as lack of physical activity can lead to an imbalance between calorie intake and expenditure. Both A and B could be considered characteristics of a sedentary lifestyle, but A is more directly related to the metabolic rate.
So, depending on the context of the question, both A and B could be appropriate answers.