3. Strontium-90 is produced during the nuclear fission of uranium-235 and is part of nuclear fallout created by weapons testing. If the half-life of Sr-90 is 28 days, how long will it take for grass contaminated with Sr-90 to be safe (<2 percent of the starting radioactivity) for cattle to eat?
A. 158 days
B. 28 days
C. 1 year
D. 158 years​