According to the passage, the case of the Penan foragers' cultivation of the sago palm fails to indicate that
A． a co-evolutionary argument is unnecessary to establishing that rain-forest adaptation is the effect of human intervention B． plant-species inimical to carbohydrate-rich species would continue to thrive without the practices of foraging peoples C． the "thinning" effect produced by the Penan's foraging practice is responsible for the sago palm's long-term survival D． the Penan have justification for abandoning the exploitation of sago palms or relocating to more carbohydrate-rich areas E． rain forests may evolve as a result of the quantity and availability of extant carbohydrate-rich species