Element a Decays into Element B with a Half-Life of 700

Multiple Choice

Element A decays into element B with a half-life of 700 million years. An astronomer finds a rock that she knows was formed originally without any B present. But the rock now contains both A and B, with the A+B portion of the rock being 1/8 A and 7/8 B. How old is the rock?

A) 350 million years
B) 700 million years
C) 1.4 billion years
D) 2.1 billion years