The diagram shows the rate of decay of potassium-40, a radioactive element. Imagine two rocks. One rock contains nearly all of its original potassium-40. In a second rock, most of the potassium-40 has decayed. Which statement about the first rock is TRUE?
A. It is a metamorphic rock.
B. It is older than the second rock.
C. It is harder than the second rock.
D. It is younger that the second rock.