[单选题]
94.24: A sample of radioactive material has an initial actiic5.t xt imq5.vity 2redm h9;b-anof 10,000 counts/minute. 30 minutes later, its actie mr-a db9h2n;vity is 2,500 counts/minute. The half-life of the material is:
A. 7.5 minutes
B. 10 minutes
C. 15 minutes
D. 20 minutes
E. 22 minutes
