[单选题]
A radioactive substance with a half-life of 3.0 days ha7mqf4 x :rzq):9mn itms an ini1j g0a602 z mclb:ko vpx4tah8tial activity of p8g4o:a010c zml tajvb h k6x2$0.24 \; Ci$. What is its activity after 6.0 days?
A. $0.12 \; Ci$
B. $0.48 \; Ci$
C. $0.06 \; Ci$
D. none of the given answers
