[单选题]
A Thallium source wa-u dxk:;lr 4 h vcnk.r.9 8+w5uhndtlcn + twia)hd9b; n0pyj9s certified at $10 \; \mu Ci$ 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. $1.0 \; \mu Ci$
