The table below shows the half-life of an element: Half-Life Data Element Half-Life Uranium-235 700 million years Scientists have found that the Uranium-235 content of an unknown prehistoric sample was reduced to one-fourth. What best describes the identity of the sample? A stone artifact that formed 700 million years ago A mammoth that died 1,400 million years ago A cliff that formed 1,400 million years ago A plant that died 700 million years ago