Radium-226 Is a Common Isotope on Earth, but Has a Half-Life

Multiple Choice

Radium-226 is a common isotope on Earth, but has a half-life of about 1600 years. Given that Earth is some 5 billion years old, why is there any radium at all?

A) Radium-226 is one of several self-transmutating isotopes of the elements of the periodic table and is able to replenish itself so that it is never depleted.
B) Radium-226 and Radium-218 undergo a series of transmutation reactions of alpha and beta decay to repeatedly become one another approximately every 1600 years.
C) Radium-226 is a "daughter" isotope and the result of the radioactive decay of uranium.
D) Radium-226 and Astatine-218 are converted back and forth via transmutation to one another at each of their respective half-life cycles.