[单选题]
A Thallium source was certifiy8efc9u7km6efxh r4 fl.7/fa ed atuxxl *6exn/ud9-ggz-* iaub . $10 \; \mu Ci$ ten years ago. What is its activity now if the half-life is 3.7 years?
A. $4.7 \; \mu Ci$
B. $3.3 \; \mu Ci$
C. $1.5 \; \mu Ci$
D. $1.0 \; \mu Ci$