It can be inferred from the passage that the "existing research" found which of the following about animals' play behavior? A. Most animal species show significant sex differentiation when engaged in dyadic play. B. Dominant animals often appear to adjust their behavior during play in order to encourage subordinate partners to play with them. C. Larger D. Few animal species that engage in dyadic play adhere closely to the 50:50 rule during play encounters. E. Some animal species show much more asymmetry during dyadic play than other species do.