Iodine-131, T_1/2 = 8 $\beta$ Particles Per Day, How Long Will It Take for the Treatment

Iodine-131, t_1/2 = 8.0 days, is used in diagnosis and treatment of thyroid gland diseases. If a laboratory sample of iodine-131 initially emits 9.95 × 10¹⁸ $\beta$ particles per day, how long will it take for the activity to drop to 6.22 × 10¹⁷ $\beta$ particles per day?

A) 2.0 days
B) 16 days
C) 32 days
D) 128 days
E) none of the above