HELP QUICK ASAP> The graph shows calculations for potassium-argon dating.

Potassium Argon Dating graph. The x axis is Billions of Years in increments of 1.3. The y axis is Remaining Portion of Potassium, measured in fractions starting at StartFraction 1 over 16 EndFraction, each entry double the previous one. 0 Billions of Years, StartFraction 1 over 1 EndFraction. 1.3 Billions of Years, StartFraction 1 over 2 EndFraction. 2.6 Billions of Years, StartFraction 1 over 4 EndFraction. 3.9 Billions of Years, StartFraction 1 over 8 EndFraction. 5.2 Billions of Years, StartFraction 1 over 16 EndFraction.

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.