When Captain Cook ________ Hawaii in 1776, the islands may have been peopled by as many as eight to ten thousand inhabitants.
句意： 当库克船长在 1776 年发现夏威夷时， 这些岛屿可能已经有八千到一万居民了。 主句是现在完成时， may have been 表示推测过去可能处于的某种状态， 从句用一般过去时。因此， 本题的正确答案为 C。