Concept:Tropical rainforests contain mixed tree species, not large uniform stands of a single species.Explanation:Lumbering is costly in tropical forests because trees of the same species are widely scattered.Loggers must move over large areas to find and extract valuable trees.Absence of pure, uniform stands reduces efficiency and increases time, labour, and transport cost.So the main reason for high lumbering cost is the lack of pure stands of trees.Answer:C. Absence of pure stands of trees