[单选题]
A Thallium source was 4/ygbni wt:lt 6m.v9g+r3yr ar 4 nkv.certio m ui*/nxiyeq/,yokn; 1(:nefied 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. $1.0 \; \mu Ci$
