(A)Anthropologists
(B)Agricultural experts
(C)Medical researchers
(D)Economists
17.Where in the passage does the author describe why the appearance of beads may change?
(A)Lines 3-4
(B)Lines 6-8
(C)Lines 12-13
(D)Lines 20-22.
Question 18-31
In the world of birds,bill design is a prime example of evolutionary fine-tuning.Shorebirds such as oystercatchers use their bills to pry open the tightly sealed shells of their prey,hummingbirds have stiletto-like bills to probe the deepest nectar-bearing flowers,and kiwis smell out earthworms thanks to nostrils located at the tip of their beaks.But few birds are more intimately tied to their source of sustenance than are crossbills.Two species of these finches,named for the way the upper and lower parts of their bills cross,rather than meet in the middle,reside in the evergreen forests of North America and feed on the seeds held within the cones of coniferous trees.
The efficiency of the bill is evident when a crossbill locates a cone.Using a lateral motion of its lower mandible,the bird separates two overlapping scales on the cone and exposes the seed.The crossed mandibles enable the bird to exert a powerful biting force at the bill tips,which is critical for maneuvering them between the scales and spreading the scales apart.Next,the crossbill snakes its long tongue into the gap and draws out the seed.Using the combined action of the bill and tongue,the bird cracks open and discards the woody seed covering action and swallows the nutritious inner kernel.This whole process takes but a few seconds and is repeated hundreds of times a day.
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