17. The author of the passage is primarily concerned with

　　discussing different approaches biologists have taken to testing theories about the distribution of plants in Hawaii

　　discussing different theories about the transport of plant seeds to Hawaii

　　discussing the extent to which air currents are responsible for the dispersal of plant seeds to Hawaii

　　resolving a dispute about the adaptability of plant seeds to bird transport

　　resolving a dispute about the ability of birds to carry plant seeds long distances

　　18. The author mentions the results of flotation experiments on plant seeds most probably in order to

　　support the claim that the distribution of plants in Hawaii is the result of the long-distance dispersal of seeds

　　lend credibility to the thesis that air currents provide a method of transport for plant seeds to Hawaii

　　suggest that the long-distance dispersal of seeds is a process that requires long periods of time

　　challenge the claim that ocean currents are responsible for the transport of plant seeds to Hawaii

　　refute the claim that Hawaiian flora evolved independently from flora in other parts of the world

　　19. It can be inferred from information in the passage that the existence in alpine regions of Hawaii of a plant species that also grows in the southwestern United States would justify which of the following conclusions?

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