Strontium-90 Is Produced in Nuclear Fission Reactors

Strontium-90 is produced in nuclear fission reactors.If ingested it can replace calcium in the bones.The half-life of ⁹⁰Sr is 27.7 years.If the activity of ⁹⁰Sr in the bones of an exposed person were 90 disintegrations per second,how long would it take the activity of ⁹⁰Sr to decrease to 9.2 disintegrations per second?

A) 91 years
B) 67 years
C) 46 years
D) 54 years
E) 78 years