The ivory-billed woodpecker, once the largest woodpecker in North America at a striking 20 inches tall, has moved in and out of the extinct category, with scientists consistently unable to find solid proof for the bird to still exist.
for the bird to still exist
for the bird as still existing
for the bird's continued existence
that the bird still exists
of the bird and its continued existence
C - 定语, 说的是proof的特征; D - 同位语从句,说的是proof的内容。选D,因为是内容。
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