Rock samples have been dated to be 10,000 years ago. -- Rock samples have analyzed by sceientist to be at least 10,000 old.
Rock samples have been dated at 10,000 years ago -- This is not possible. First, science was not that fancy 10,000 years ago to be able to know the age of rock sample. Second, even it is the case, you cannot use past participle and you have to change to past tense.
Fossils of the arm of a sloth found in Puerto Rico in 1991, and dated at 34 million years old, made it the earliest known mammal of the Greater Antilles islands.
此题选这个选项 sloth, found in Puerto Rico in 1991, have been dated at 34 million years old, making the sloth the earliest known mammal on
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发表于 2007-12-11 14:34:00
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