If the half-life of a radioactive substance is 500 million years and you have 192 atoms of it, how many half-lives will have passed by the time that only 3
atoms of the substance remain?
A. 4
B. 5
C. 6
D. 2.

Paleontologists have found plant fossils deep within the layers of the north polar region. these leaf fossils have been labeled as to the rock layers they were found in, with the lower numbers being layers close to the earth's surface and higher numbers representing deeper layers. according to the numbers of these four leaf fossils, what might you infer?