The graph shows calculations for potassium-argon dating.

Which statement explains what geologists can learn from the graph?

They can estimate that all the potassium will have decayed within 6.5 billion years.
They can tell that 1/8 of the argon gas will be left after 3.9 billion years.
They can determine that a rock is 1.3 billion years old if 1/2 of the potassium has decayed.
They can see that potassium’s half-life varies from every 1.3 billion years to every 5.2 billion years.

THE ANSWER IS C