Answer:
C. 40 s
Explanation:
The original 30 atoms is reduced to 15 atoms at 40 s
those 15 atoms are reduce to 7.5 ( call it 8) atoms at 80 s
those 8 are reduced to 4 at 120s
those 4 are reduced to 2 at 160 s
those 2 are reduced to 1 at 200 s
