Strontium is chemically similar to calcicum and can replace calcium in bones. The radiation from `Sr^(90)` can damage bone narrow marrow where blood cells are produced, and lead to serious health problems. The time taken for 99.99% of a sample of `Sr^(90)` is `lambda "for" Sr^(90)=0.23yr^(-1): `
A. 10yr
B. 200yr
C. 40yr
D. 400yr
