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

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 these choices is correct.