http://www.google.com/#q=half-life%2Bof%2B1.3%2Bbillion%2Byears
http://www.google.com/#q=carbon+14+half+life
_____ is useful for dating very old rocks because it has a half-life of 1.3 billion years.
A. thorium-232
B. potassium-40
C. uranium-235
D. carbon-14
is the answer D?
2 answers
potassium-40