[单项选择]Since the Hawaiian Islands have never been connected to other landmasses, the great variety of plants in Hawaii must be a result of the long-distance dispersal of seeds, a process that requires both a method of transport and an equivalence between the ecology of the source area and that of the recipient area.
There is some dispute about the method of transport involved. Some biologists argue that ocean and air currents are responsible for the transport of plant seeds to Hawaii. Yet the results of flotation experiments and the low temperatures of air currents cast doubt on these hypotheses. More probable is bird transport, either externally, by accidental attachment of the seeds to feathers, or internally, by the swallowing of fruit and subsequent excretion of the seeds. While it is likely that fewer varieties of plant seeds have reached Hawaii externally than internally, more varieties are known to be adapted to external than to internal transport.
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A. Why does successful long-distance dispersal of plant seeds require equivalence between the ecology of the source area and that of the recipient area
B. Why are more varieties of plant seeds adapted to external rather than to internal bird transport
C. What varieties of plant seeds are birds that fly long distances most likely to swallow
D. What is a reason for accepting the long-distance dispersal of plant seeds as an explanation for the origin of Hawaii flora