The graph below shows the radioactive decay of a material.

The graph shows 'Time in billion years' on the x axis and 'Percent of original mass remaining' on y axis. On the x axis the points 0, 2, 4, 6, and 8 are marked. On the y axis points 25%, 50%, 75%, and 100% are marked. A smooth curve joins the points (0, 100%), (2, 25%), (4, 12.5%), (6, 6.25%), (8, 3.125%).

If the original mass of the radioactive material was 500 grams, after how many billion years will its mass become 125 grams?

2 billion years
4 billion years
6 billion years
8 billion years