Potassium-40 (K-40) decays into argon-40 (Ar-40). After 1.25 billion years, 20 grams of K-40 would be reduced to 10 grams. How much of the original K-40 would remain after 5 billion years?

A. 0 grams
B. 1.25 grams
C. 2.5 grams
D. 5 grams