A radioactive medical tracer is prepared for a diagnostic scan. The graph shows how the activity of a sample of this tracer varies with time:
What was the initial number of active nuclei in the sample?
2.5×1052.5 \times 10^52.5×105
6.9×1076.9 \times 10^76.9×107
1.7×10111.7 \times 10^{11}1.7×1011
2.5×10112.5 \times 10^{11}2.5×1011
