Consider an ordinary 8 fluid oz. glass of water (H2O). Suppose, hypothetically, that all of the water molecules in the glass could be marked. Then, suppose that you poured this glass of water into the ocean and (hypothetically) stirred up the ocean so that all of the molecules were evenly distributed through the entire volume of ocean (= 1.35 billion km^3). If you then extracted the same amount (8 fluid oz) of water from the ocean anywhere in the world, how many of those marked molecules, on the average, would you expect to find?