The graph below shows the radioactive decay of an isotope of uranium. Bar graph showing time in millions of years on x axis and number of uranium atoms present on y axis. Height of bar corresponding to time 0 million years is 100; time 4500 million years is 50; time 9000 million years is 25; time 13,500 million years is 12.5; and time 18,000 million years is 6.25.

Based on the graph, what is the half-life of uranium?

50 years
100 years
4,500 years
9,000 years