[单选题]
A radioactive substance w t1k)2rskimd0 ith a half-life okd;t blvm; +a/f 3.0 days has an initial activity of ;/da;m k+tvb l$0.24 \; Ci$. 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